No smaller a celebrity than Cary Grant once wandered Winfield House, the neo-Georgian mansion built for the woman he would marry, Barbara Hutton, which is now the residence of the US Ambassador in Regent's Park. But the guest waking upstairs yesterday was a bigger star even than he.
Even by the standards of Barack Obama, who in two years has gone from junior US senator to leader of the free world, the day ahead promised to be testing. By dusk there would be barely a London landmark he had not entered or a world leader he had not met.
His diary entry for the day began: "7.45am – "POTUS/FLOTUS depart for 10 Downing Street", referring to the President and First Lady of the United States. It ended: "10pm (or later) – POTUS departs for Winfield House." In between, he had to face other leaders, the British press, the British leader of the Opposition and the British Queen.
Yet the eagle-eyed reporters at Downing Street to witness the first couple arriving for breakfast with Gordon and Sarah Brown will have seen what American voters spotted last year. This man did not just smile at the right moments, he showed off that special confidence that is not of the American-arrogant variety. It is easy calm. True, there was a little awkwardness at the doorstep as the leaders jostled with their wives to get in the right order for the photographers. But when the door swung open, it was Mr Obama's arm that went around Mr Brown's shoulder to guide him across the threshold.
Mrs Obama had privately confessed to nervousness about meeting the Queen later in the day but at Downing Street, there was no sign of it as she followed her hosts on a brief tour.
Either the Obamas and the Browns are fine actors, or they genuinely click. When their conversation turned to dinosaurs, it was not in pursuit of agreement on Darwin versus creationism but a shared moment between parents about their children's preferred toys.
To the delight of assembled British officials, the clicking seemed to extend to the nations too and the "special relationship". No sooner had the two men departed for the Foreign Office for a press conference, then the vital words were spoken. Mr Obama said that they both "greatly value the special relationship between our nations". He went on: "the United States and the United Kingdom have stood together through thick and thin, through war and peace, through hard times and prosperity."
Mr Obama knows when to be grave and there may be more moments for that at the G20 meeting today. But before the press, he deployed his talentfor quick humour, not least when asked who he favoured for last night's World Cup qualifier at Wembley. Footballs in the US, after all, look more suited for rugby (or turning on nuclear warheads.)
"The last thing I'm going to do is wade into European football," he quipped flashing a smile brighter than the spring sky outside. "That would be a mistake. I didn't get a briefing on that, but I sense that would be a mistake."
One compliment perhaps would have been better unsaid: that Mr Brown had been the first European leader to visit him in Washington and the first to host him in Europe. This might almost have been designed to irritate the leaders of France and Germany, Nicolas Sarkozy and Angela Merkel, who get jealous about such things and who, during the afternoon, held a small press conference of their own, a jarring departure from accepted summit protocol.
But Mr Obama was unaware if not of the Sarkozy-Merkel show, then certainly of the surging back and forth of thousands of angry protesters all the way from the Bank of England to Trafalgar Square. He had a schedule to keep, most of it back at Winfield House, guests he had to see: Dmitry Medvedev of Russia and Hu Jintao of China.
It was between discussions with these two leaders on subjects ranging from cutting nuclear arsenals to containing North Korea that he had another 30-minute commitment to honour: a parley with David Cameron.
It is expected of a President to see the leader of the Opposition. Such figures, after all, have on occasion become Prime Minister later on. "An extremely personable human being and someone it is easy to get on with and strike up a relationship with," Mr Cameron said of the US leader.
No time, it seemed, for Nick Clegg, whose office furiously denied reports that he had blown his top over being left off the Obama roster.
As the sun began to set on the President's first big day in London, he and Michelle had to get ready for the biggest engagement of all – meeting the Queen.
The First Lady waved to onlookers as she entered Buckingham Palace, to be greeted not by the Queen right away but by the Master of the Household, Air Vice-Marshal David Walker. From there, the first couple were ushered to a room within the Queen's private apartments without advisers, aides or palace staff and beyond the prying eyes of the press. Mr Obama was the 12th US President the Queen had met.
Prince Philip displayed his uncanny knack for leaving his mark on a state occasion, issuing a few forthright observations. As the first couple exchanged small talk with the Queen about their gruelling schedule, the prince swung into action, declaring: "You're just trying to stay awake!"
Then as President Obama listed his meetings with Gordon Brown, the Russian president and David Cameron, Prince Philip cut in again, asking: "Can you tell the difference between them?"
Gaffes aside, the private meeting was a special privilege for a special guest. As the heads of state of Britain and the United States bantered upstairs with their spouses, 19 other leaders were coming in slightly smaller limousines for the mildly less exciting prospect of canapés and cocktails with the Queen and Duke of Edinburgh.
If any were miffed, they were not about to advertise it. They would have the working dinner at Downing Street last night to voice whatever resentments or reservations they were feeling before the start of the G20 proper, though nothing too sour, surely, to spoil the spread cooked by Jamie Oliver that included Childwickbury goat's cheese, Welsh lamb and Bakewell tart.
Chafe at it in private if they like, but the Group of 19 are just not on the same plane as the Group of One. That would be world superstar Barack Obama.