Imagine being Sir Keir Starmer.

In less than seven days, he has not only become the most successful politician at the ballot box since Tony Blair thanks to his landslide victory, but then jetted off to Washington for a huge international summit and his first audience with the US president in the Oval Office.

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It has been a whirlwind few days, and it’s fair to say that as I watched him sitting in that famous office with President Biden and sat down for our first interview since he became prime minister, he was having a moment to take it all in.

On the personal, there was a hint of how it feels to win from opposition when you began the job looking like you were 6-0 down, and what it means to him as one of the few prime ministers from a working-class background that this country’s had.

When I asked him how it felt to be called prime minister by the president, Sir Keir smiled and said: “It felt very good.”

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Later in our interview, he also became a little emotional when I asked him what his late parents – Jo and Rodney – would have made of seeing their son walk through the door of Number 10.

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“They would have loved that,” said Sir Keir, as he smiled again and looked a little teary. “There’s a real part of me that just wishes that they could have been there, because that would been very special for them.

“Obviously, they’re not with us anymore. They won’t have that moment. But I can’t pretend to you that I haven’t thought about that.”

At the time of the Platinum Jubilee, Sir Keir spoke of how the “proudest day” of their lives was witnessing their son receive a knighthood at Buckingham Palace.

Was it bittersweet then for him to succeed to the highest office in this country and for them not to see it?

“Not bittersweet but, you know, I would have loved them to have been there and they weren’t there,” he said.

“And Vic [his wife] obviously lost her mum in the Labour leadership contest. So both of us had a thought, and a discussion about that.

“We would have loved our parents to be there. No, they weren’t, but we smiled because we know what they would have thought had they been there.”

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With the new office has come a confidence too. Sir Keir Starmer as prime minister is equivocating far less than he did when he was the leader of the opposition trying to get into power.

He seemed reluctant to put much substantial policy into his manifesto, afraid to box himself in or frighten voters off. But now he has the office of PM, he seems far more willing to put his views on record.

When I asked him about President Biden’s cognitive ability in light of the storm around whether he should run for a second term, the prime minister was forthright.

“We worked at pace and he was on good form with me,” said Sir Keir.

When I asked him, twice, whether the criticism of the president was misguided, he told me, simply, “yes”, adding: “My own personal view is he was on good form and we went through a lot.

“I was very keen obviously to discuss Ukraine, but there were many other issues that we, we got through last night.”

With a successful visit to the White House, hugs from President Macron, and meetings with Chancellor Schloz and President Zelenskyy to name a few of those Sir Keir has managed to greet in the past 48 hours, the trip will go down as a triumph in Number 10, with aides particularly pleased – as the prime minister noted in our interview – that the bilateral meeting with President Biden overran.

But he and his team know too that is likely to be the only trip he is ever going to do without baggage, and hints of what was to come ran throughout the interview.

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On defence spending and the future of NATO in the shadow of a potential Trump presidency, Sir Keir swerved US domestic politics, but he also acknowledged that Europe would have to increase spending on defence.

“In terms of the contribution of all NATO allies, you’re right to say that we’re all going to have to put more in. And that was a big theme in the council yesterday, particularly in relation to Ukraine.”

But when it comes to his own commitment to increase defence spending to 2.5%, he simply, despite my best efforts, would not be drawn on a timescale.

I put it to him that without it, his pledge was an ambition rather than a commitment, to which he strenuously disagreed.

“It is serious [and] why I want to set out a roadmap to it within our fiscal rules,” said the prime minister. “I think what would be unserious would be to simply pick an arbitrary date.

“I’m not going to put a date on it because it’s going to be within our fiscal rules.”

But he rejected the notion he was not putting his money where his mouth is, saying: “What we’ve had for the last 14 years before the change of government was a series of arbitrary dates and lack of funding for meeting those targets, which then were missed over and over again.

“We’ve changed, turned our back on that way of doing politics. This is a different politics. Serious, realistic. We’ve made the commitment that we will reach.”

How he squares the 2.5% pledge as a “commitment that we will reach” without pinning it to his first term of government is a contradiction.

If he intends to reach the target, he has to make good on the pledge in his first term, and I suspect this will be a theme that keeps running through his defence review – and his premiership – if he refuses to commit in the first term of office.

But you can understand too why Sir Keir is being cautious as he surveys the in-tray and considers where precious resources should be spent.

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The most pressing problem he has is prisons, which are in crisis over a shortage of places.

Sir Keir told me: “I can’t tell you how concerned I am about this. I knew it was a problem. It is. We now see a much worse problem than we thought it would be.

“I am genuinely shocked by what I’ve seen, particularly as someone who’s worked in criminal justice for a long time. It is a basic function of government to have enough prison places for those that the courts are sending to prison.”

His government will make an announcement on this, perhaps as early as this week, amid talk that the system is going to have to release more prisoners – up to perhaps 20,000 in coming months – to ease the pressure.

“I’m shocked to have to pick up this problem,” he told me.

But these are now his problems. From prisons to NHS waiting lists, from defence budgets to child poverty, and countless matters in between.

The first week has been a whirlwind for a man, and a government, that wants to show it will hit the ground running.

But the prime minister is in no doubt of the scale of the task ahead. For now, he can blame the difficulties on the past administration. Over time, they will be on him.

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