Travelling over to Egypt, flanked by his national security advisor Jonathan Powell, the prime minister told me it was a "massive moment" and one that is genuinely historic.
US President Donald Trump moved decisively last week to end this bloody war, pushing Israeli Prime Minister Benjamin Netanyahu and Hamas into a ceasefire as part of his 20-point peace plan.
In the flurry of the following 48 hours, Mr Starmer and another twenty or so leaders were invited to Egypt to bear witness to the signing of this deal, with many of them deserving some credit for the effort they made to bring this deal around, not least the leaders of Qatar, Egypt and Turkey, who pressed Hamas to sign up to this deal.
On Monday, the remaining 20 living hostages are finally set to be released, along with the bodies of another 28 who were either killed or died in captivity, and aid is due to flow back into a starving Gaza.
Some 1,200 Israelis were killed on 7 October 2023, with another 250 taken hostage. In the subsequent war, most of Gaza's two million population has been displaced. More than 67,000 Gazans have been killed, according to Palestinian health officials
Then, the signing ceremony is due to take place on Monday afternoon in Sharm el-Sheikh. It will be a momentous moment after a long and bloody war.
But it is only just the beginning of a long process to rebuild Gaza and try to secure a lasting peace in the region.
The immediate focus for the UK and other nations will be to get aid into Gaza with the UK committing £20m on Monday for water, sanitation and hygiene services for Gazans.
But the bigger focus for the UK and other European allies is what happens after the hostages are released and Israel withdraws its troops.
Because what happens next is a much bigger and more complicated task: rebuilding Gaza; turning it into a terrorist-free zone; governing Gaza - the current plan is for a temporary apolitical committee; creating an international stabilisation force and all the tensions that could bring about - which troops each side would allow in; a commitment for Israel not to occupy or annex Gaza, even as Netanyahu makes plain his opposition to that plan.
The scale of the challenge is matched by the scale of devastation caused by this brutal war.
The prime minister will tomorrow set out his ambition for the UK to play a leading role in the next phase of the peace plan.
Back home the UK is hosting a three-day conference on Gaza's recovery and reconstruction.
Last week, France hosted European diplomats and key figures from Middle Eastern countries, Egypt, Jordan, Saudi Arabia and Qatar and later this week, the German chancellor is hoping to organise a conference on the reconstruction of Gaza with the Egyptians.
But in reality, European leaders know the key to phase two remains the key to phase one: and that's Donald Trump.
As one UK figure put it to me over the weekend: "There is lots of praise, rightly, for the US president, who got this over the line, but the big challenge for us post-war is implementing the plan. Clearly, Arab partners are concerned the US will lose focus".
The prime minister knows this and has made a point, at every point, to praise Mr Trump.
His cabinet minister Bridget Phillipson learned that diplomatic lesson the hard way on Sunday when she was publicly lambasted by the US ambassador to Israel Mike Huckabee for suggesting to my colleague Trevor Phillips that the UK "had played a key role behind the scenes" and failed to mention Mr Trump by name.
"I assure you she is delusional," tweeted Governor Huckabee. "She can thank @realDonaldTrump anytime just to set the record straight".
On Monday, leaders will rightly be praising Mr Trump for securing the breakthrough to stop the fighting and get the remaining hostages home.
But this is only the beginning of a very long journey ahead to push through the rest of the 19-point plan and stop the region from falling back into conflict.
Britain has, I am told, been playing a role behind the scenes. The PM's national security adviser Mr Powell was in Egypt last week and has been in daily touch with his US counterpart Steve Witkoff, according to government sources. Next week the King of Jordan will come to the UK.
Part of the UK's task will be to get more involved, with the government and European partners keen to get further European representation on Trump's temporary governance committee for Gaza, which Tony Blair (who was not recommended or endorsed by the UK) is on and Mr Trump will chair.
The committee will include other heads of states and members, including qualified Palestinians and international experts.
As for the former prime minister's involvement, there hasn't been an overt ringing endorsement from the UK government.
It's helpful to have Mr Blair at the table because he can communicate back to the current government, but equally, as one diplomatic source put it to me: "While a lot of people in the Middle East acknowledge his experience, expertise and contact book, they don't like him and we need - sooner rather than later - other names included that Gulf partners can get behind."
On Monday it will be the US, Egypt, Qatar and Turkey that sign off on the peace plan they directly negotiated, as other Middle Eastern and European leaders, who have flown into Sharm el-Sheikh to bear witness, look on.
But in the coming days and weeks, there will need to be a big international effort, led by Mr Trump, not just to secure the peace, but to keep it.
The 20 living captives, who are all men, have been held for more than two years after they were among 251 people kidnapped when Hamas-led militants launched a surprise attack on Israel on 7 October 2023.
Gaza latest: Don't make a 'sick display' of hostages, Hamas warned
The bodies of another 28 hostages who were killed in captivity are also expected to be returned to Israel.
Exact details of the handover are still unclear but this is what we know.
When is the deadline?
Hamas have until 10am UK time to release the hostages as part of Donald Trump's 20-point peace plan.
What time will they actually be freed?
There have been mixed messages about the exact timings and Israel's prime minister Benjamin Netanyahu said Israel was ready to "immediately" receive the captives.
US vice-president JD Vance said earlier on Sunday the hostages would be released "any moment now".
But the most concrete statement we've heard comes from Israeli government spokeswoman Shosh Bedrosian, who said they will be released "early Monday morning".
Hamas has previously indicated it would take more time to hand over the bodies of the hostages who have died to Israel.
How will the handover work?
Israeli government spokeswoman Shosh Bedrosian said the 20 hostages are expected to be released together at one time to the Red Cross.
She said they will be taken in "six to eight" vehicles without "any sick displays by Hamas", referring to the previous release ceremonies staged by the militant group.
The hostages will then be driven to forces inside Israel-controlled parts of Gaza before they are transferred to southern Israel, where they will reunite with their families.
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"Israel is ready if a living hostage requires any urgent medical attention, they will be brought to a medical facility immediately," she said.
She said the "entire nation of Israel" will embrace those killed in captivity and "ensure proper Jewish burials".
The US president is expected to arrive in Israel on Monday morning and will meet with families of the hostages, according to a schedule released by the White House.
Where will they spend their first nights of freedom?
Pictures released by the Israeli government show the light airy rooms where the hostages will be welcomed.
The comfortable chairs, big television, and bedrooms with clean sheets and baskets full of toiletries will be a world away from the dark Hamas tunnels where they have been kept in Gaza for the past 734 days.
Sky News was given special access to one of the teams in the Rabin Medical Centre in Petah Tikva, a city northeast of Tel Aviv, which is preparing to receive them.
Director of Nursing Dr Michal Steinman said the hostages will be given their own private room, where a gift basket filled with items such as a teddy, a blanket, slippers and a phone charger awaits them.
While the bear might make you think a child is about to arrive, this room will soon be welcoming one of the 20 Israeli hostages believed to be alive in Gaza.
With phase one of Donald Trump's peace plan now under way, an entire nation is holding its breath for the return of the hostages, not least the medical teams preparing to receive them.
Gaza latest: Israel prepares for hostages' release
Sky News was given special access to one of the teams in the Rabin Medical Center in Petah Tikva, a city north-east of Tel Aviv.
It was sobering and emotional, but also inspiring, talking to its doctors and nurses as they showed us around what one calls the "homecoming unit".
Director of Nursing Dr Michal Steinman took us into the light airy rooms where hostages will be allowed to recover at their own speed in private, choosing when and for how long they emerge, slowly reengaging with a world they've not known for two years.
She explained that each of the hostages - who are all men - will be given their own private room, where a gift basket filled with thoughtful items such as a teddy, a blanket, slippers and a phone charger awaits them.
The teddy is there to help bring comfort to the freed captives.
"Our research says each one of us has a child inside," Dr Steinman told me. "We need something to pet and feel soft, and reassure them after the lack of senses for such a long time."
Phones, she said, will be provided by the army.
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The families will also bring items from home to make the area feel more familiar to their loved ones as they slowly adjust to freedom.
The men will also have access to other areas, including a private living space where they can spend time with loved ones or greet any visiting dignitaries. Their families will also be provided with rooms to stay in, as well as an area for the children of the hostages when they visit.
Medical equipment is kept in dedicated treatment rooms as part of an effort to make the rooms feel more like accommodation than a hospital.
While the unit is pristine and ready for the new arrivals, it has previously been used to house other hostages released by Hamas.
Staff shared anecdotes revealing what may lie ahead. Dr Steinman told us of one released hostage who had had trouble not with sleeping, but with waking up.
"When I opened my eyes," they had told her, "I was thinking that I'm still in a dream because there's no way that I opened my eyes and I'm not in the tunnel. I thought, 'it's a dream inside a dream'."
The hostages, she said, "can't believe for the first moments they're not in other place."
Dr Steinman found another freed captive "stuck" and standing still after opening the refrigerator.
"I told him, 'It's hard for you to choose?'," she explained. "And he said, 'I'm just amazed at the colours. All I've seen for 100 days is black, white and brown'."
The professor reinventing 'hostage medicine'
For the head of the centre, Professor Noa Eliakim-Raz, and her team, the return of the hostages will be the culmination of two years of painstaking work.
They have effectively reinvented what they call 'hostage medicine', learning from the treatment of groups of hostages received during this war.
She is a serious and dedicated clinician. With professional precision, she told me of the challenges ahead, including the life-threatening risks of mistreating malnourished hostages held for so long underground.
Then she gave us a glimpse into the human side of their work.
"All the team, we've prepared for so long, I mean really, we've been in this for two years and all the time, we're preparing and ready," she said. "This ward that you saw is ready every day."
How does she feel as the hostages' arrival draws near?
"I feel very grateful, and I think that's the strongest emotion, to be part of this," she said.
Clearly moved, Professor Noa had to pause and collect her emotions, her eyes welling up when asked what she'd be thankful for most.
"I think being part of a small step," she began, before pausing again. "A small step of making them feel hugged again and trusting the system."
It will, she said, be a big relief when it's over.
Professor Noa is writing a first-of-its-kind multi-disciplinary protocol for treating long-term hostages, literally rewriting the book on how to return them to normality.
Her department did not exist before October 7. In the two years since its inception, it has pioneered a form of treatment involving many different disciplines to maximise the chances of recovery.
The Rabin Medical Center's staff believe the lessons they've learned will benefit doctors around the world in future.
But they hope never to have to use them on Israelis again.
The education secretary told Sunday Morning With Trevor Phillips, while recognising the "critical role" of the US, that Sir Keir Starmer's presence at a signing ceremony for the ceasefire deal in Egypt on Monday "demonstrates the key role that we have played".
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She did not say exactly what the UK's role in the ceasefire, largely attributed to Donald Trump, is or was.
But she added: "We have played a key role behind the scenes in shaping this.
"It's right that we do so because it's in all of our interest, including our own national interest, that we move to a lasting peace in the region.
"These are complex matters of diplomacy that we are involved in. But we do welcome and recognise the critical role that the American government played in moving us to this point."
However, the US ambassador to Israel, Mike Huckabee, reposted the Sky News clip of Ms Phillipson's comments and said: "I assure you she's delusional.
"She can thank Donald Trump anytime just to set the record straight."
Mr Huckabee was part of the negotiating team for the peace deal, with Mr Trump calling the former Arkansas governor "AMAZING" as he said he "worked so hard, and did so much, to bring about peace in the Middle East".
In August, Mr Huckabee said the UK and other European nations who said they would declare a Palestinian state were "having the counterproductive effect that they probably think that they want".
Israel: UK did not play key role in peace deal
Israel's deputy foreign affairs minister, Sharren Haskell, told Sky News the UK played "the opposite" of a key role in the peace deal after the Palestinian state declaration, which eventually happened in September.
She accused the PM's initial threat "at a very sensitive time" in July of having "pushed Hamas to embolden their position and to refuse a ceasefire two months ago".
"I think that right now, the quiet that was given during the negotiation, and to President Trump, had probably played a bigger role than what the government had done two months ago," she said.
"The message that the UK government has sent Hamas was the message that: the longer they continue this war, they will be rewarded.
"I mean, you must understand that when a terrorist organisation is thanking you. You are on the wrong side of history."
It's understood the prime minister has underlined Mr Trump's key role in securing this deal throughout the process. Publicly, he praised Mr Trump twice in his press conference in India on Thursday.
Read more:
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Ms Phillipson stood by the decision to declare a Palestinian state, saying it was "the right thing to do".
Husam Zomlot, the Palestinian ambassador to the UK, told Sky News it was a "moment of leadership" for the UK to declare a Palestinian state, and a "responsibility to begin to correct a century of the gravest historic injustices committed against our people".
He added: "That moment three weeks ago, when the UK did recognise, is a moment when we can say that the wheels of history are turning in a different direction."
No plans for British troops on the ground
The education secretary also told Sky News the government has "no plans" to put British troops into Israel or Gaza as part of a stabilisation force after the ceasefire.
The US military will help establish a multinational force in Israel, known as a civil-military coordination centre, which is likely to include troops from Egypt, Qatar, Turkey and the UAE.
On Friday, US officials said up to 200 US troops already based in the Middle East will be moved to Israel to help monitor the ceasefire in Gaza.
The day before, President Trump announced Israel and Hamas had "signed off on the first phase" of a peace plan he unveiled last week.
Aid trucks have been gathering in Egypt to cross into Gaza after months of warnings by aid groups of famine in parts of the territory.
In Israel, the remaining hostages are due to be returned from Gaza by Hamas on Monday under the first phase of the peace plan. Twenty are believed to still be alive, 26 have been declared dead, while the fate of two is unknown.
The ceasefire agreement has been made two years after Hamas stormed Israel on 7 October 2023, killing 1,200 people and taking 251 hostages.
Israel's military offensive has killed more than 67,000 Palestinians in Gaza, according to the Hamas-run health ministry, which the UN deems reliable.
She watched as her mother Rose frantically searched for her sister Agnes for 61 days before the body was found in a hotel septic tank.
Agnes's baby Stacey was only five months old at the time her mother was murdered and in the thirteen years since, Rose has raised Stacey as her own child.
Her daughter Esther has grown to become the family spokesperson, urging the Kenyan and British governments to deliver justice.
In all this time, the British soldier accused of murdering Agnes has been living freely in the UK.
"I knew I had to take this responsibility, especially because of her daughter. She was left at five months old and is turning 14, and I always feel bad for her because it is a lot knowing what happened to her mum and knowing that the government knew and never took any action," Esther tells us, by the tree that marks her aunt's grave site in a Nanyuki cemetery.
Esther is now 21 years old, the same age Agnes was when she was killed. Instead of spending her time at university in Nairobi, she is preparing for an important meeting in London.
On Tuesday, Esther will sit with Defence Secretary John Healey and discuss on British soil the case of her aunt's murder.
"I feel excited but I am also feeling sad because at 21 years, I should not be doing this. I should be doing other things and enjoying my life but since the British government and Kenyan government failed that is why I am planning all this - to go and lobby for Agnes."
The Kenyan national prosecuting authority has finally issued an arrest warrant for the British soldier charged with murdering Agnes.
The Kenyan government has expressed intent to request his extradition, but the formal process is yet to begin.
"Although they have taken long and [are doing this] because of the pressure put on them, I think something will be done because the warrant of arrest has been issued. But again, it is a long journey ahead of us. Extradition might take as long as five years," says Esther.
"We want the guy to be extradited here so that he can be a lesson for other soldiers who think they can come to Kenya and do anything that they feel like doing because they have power."
Agnes worked as a hairdresser and sex worker at the time she was killed.
In November 2022, the Ministry of Defence banned British soldiers from soliciting sex overseas but an internal service inquiry found there have been low to moderate violations of the ban in Kenya.
Maryanne Wangui, an old friend of Agnes and leader of the sex workers alliance in Nanyuki, says that British soldiers are still paying for sex in the town, but much more discreetly.
"They are using Airbnbs and they have their pimps who they are using to bring those sex workers to their houses and they do their stuff there and pay them there. We are not protected because if a girl is killed in an Airbnb - who will cater for that death?" she asks.
The Ministry of Defence has told Sky News: "There is absolutely no place for sexual exploitation and abuse by people in the British Army.
"It is at complete odds with what it means to be a British soldier. It preys on the vulnerable and benefits those who seek to profit from abuse and exploitation."
The MoD also sent this comment on Agnes Wanjiru's murder: "Our thoughts remain with the family of Agnes Wanjiru and we remain absolutely committed to helping them secure justice.
"We understand that the Kenyan Director of Public Prosecutions has determined that a British National should face trial in relation to the murder of Ms Wanjiru in 2012. This is subject to ongoing legal proceedings and we will not comment further at this stage."
Agnes's hometown Nanyuki is the main base for the British Army Training Unit in Kenya (BATUK).
The market town was first set up as a white frontier settlement in the 1920s after the mass expulsion of Maasai from the Laikipia plateau by British colonial forces.
It became favoured by retired British soldiers who wanted to turn to agriculture and sixty years since Kenya's independence, the town still feels trapped in the past.
Public hearings last year saw dozens of Kenyans allege violations by the BATUK in Laikipia and Samburu.
Local politicians, religious leaders and village elders were among those from across the region to share their grievances with the BATUK and requested compensation from the UK government.
Esther made her aunt's case to a committee at a community hall in Nanyuki, and now she will make her case on British soil.
"Neocolonialism is still in Kenya. Even our government knew what these soldiers had done but they never took action, they do not protect their victims - their own citizens," Esther tells us, hours before her flight to the UK.
"For sure, these guys are still colonising us because why are they training here? They have their own land."