It wasn't: Six gold medals, including three in track and field within the space of an hour Saturday night, had given the country its best day at an Olympic Games since 1908.
A country accustomed to sporting disappointment could scarcely contain its disbelief.
"Don't choke on your cornflakes," said a BBC morning TV presenter, before telling viewers that Britain, population 63 million, lay third in the medal table, behind the United States (population 310 million) and China (population 1.3 billion).
British Sunday newspapers tried to outdo one another in front-page superlatives.
The Observer declared it "Britain's greatest day," and the Sunday Times — reaching for wartime resonances — said it was "Our finest Olympic hour." For the Sunday Telegraph, it was simply "Sensational."
"The six pack," said The Sun — referring, in true tabloid style, both to the gold medallists and to the washboard abs of British heptathlon champ Jessica Ennis.
Prime Minister David Cameron tweeted that Ennis' triumph was "awe inspiring." It was followed by victories in the 10,000 metres for Mo Farah and in the long jump for Greg Rutherford, in what London games organizer Sebastian Coe called "the greatest day of sport that I have ever witnessed."
"It's a bit like glugging your way through a quart of cream," said Coe, himself an Olympic track gold medallist . "It just got better and better."
On a night when Britons were glued to their TVs — the BBC said 17 million watched Farah's race — even the public broadcaster's normally cool and collected presenters overcome.
Footage released by the BBC showed the commentary team of former British athletes Colin Jackson and Denise Lewis and U.S. runner Michael Johnson acting out a brief pageant of agony and ecstasy — leaping from their chairs, waving their arms and shouting "Come on Mo!" till they were hoarse as Farah clinched the race.
What a difference a week makes. Last Sunday, newspapers were full of gloom. Mark Cavendish had failed to win the cycling road race on the first day of competition. One newspaper dismissed him as "nowhere man" and several saw it as a doomy portent for the rest of the games.
Since then, Britain has gone from skepticism about the games — which are costing taxpayers 9.3 billion pounds ($14.5 billion) — to enthusiasm, and now to outright euphoria.
There were, inevitably, also some political points to be made.
A week after one Conservative lawmaker criticized the opening ceremony as "leftist claptrap," many hailed the diversity of Britain's winners as the face of modern Britain. Rutherford is a red-headed Englishman, Ennis is a northern lass with an English mother and a Jamaican father, and Farah is a Somali-born immigrant who came to Britain at the age of eight.
Farah was indignant when asked at a press conference whether he would have preferred to win for Somalia.
"Look, mate," he said in a London accent. "This is my country."
The British medal rush continued Sunday, with Andy Murray trouncing Roger Federer for singles tennis gold and sailor Ben Ainslie winning his fourth Olympic gold in the Finn class race. For good measure, gymnasts Louis Smith and Max Whitlock took a silver and a bronze in the pommel horse, while Iain Percy and Andrew Simpson took silver in sailing's Star class.
What had appeared an unreachable target by national Olympic officials — to better the 47 medals won in Beijing — now seems a distinct possibility.
The athletes seemed as stunned as the fans. Farah said he couldn't have done it without the "incredible" lift from the 80,000-strong hometown crowd.
Earlier, rower Kat Copeland sobbed with disbelief after she and Sophie Hosking won the lightweight doubles sculls.
"We've won the Olympics," she said. "We're going to be on a stamp tomorrow!"
The Royal Mail may be regretting its promise to have stamps featuring all gold medallists on sale within 24 hours — the printing presses are working overtime.
For some, Saturday evening was so exciting that its final act came as a relief: Britain's soccer match with South Korea ended in a draw and went to a penalty shootout.
Jill Lawless can be reached at http://Twitter.com/JillLawless