Kristian Thomas's ambitious bid for an individual gymnastics all-around medal evaporated with a mistake at the end of his intensely difficult vault.
Thomas chose a tough routine knowing he needed high marks to open up a shot at a medal, but stumbled badly on landing.
"If Kristian Thomas had hit that routine, he would have been in bronze or even silver-medal position. But you go down that road and you could say 'if' to every performance.
"Kohei Uchimura is unbeaten in four years and now the Olympic champion. He has looked rough all week, then he goes out and does that - terrific."
The 23-year-old finished seventh as
Japan's three-time world champion, sealed legendary status with Olympic gold.
Uchimura beat Germany's
to gold as GB's
Purvis, like Thomas, had made confident progress through the opening half of the six-discipline event, only to fall at the beginning of his parallel bars routine.
Had Thomas produced the same vault score he had earned in qualifying - 16.200 - instead of the 14.908 he received in the final, his total would have been good enough for
a second bronze medal
of the week following on from Monday's GB team success.
His result here remains the finest by a British man in more than a century. Walter Tysall was the last Briton to win a men's all-around medal, picking up silver in 1908, when gymnastics as a sport looked markedly different.
"I'm happy," said Thomas. "If I'd landed the vault, it maybe could have been a medal but sometimes it happens, sometimes it doesn't. All my other pieces couldn't have gone any better."
Tysall's events included rope climbing but the modern all-around final tests gymnasts on the floor, pommel horse, rings, vault, parallel bars and high bar, adding their scores to produce an overall total and find the most consistent performer.
London 2012: Gymnastic gold for Japan
In the early stages inside the North Greenwich Arena, mistakes from leading contenders like American John Orozco - a low 12.566 score on pommel horse - and Germany's Fabian Hambuchen - 13.266 on the same apparatus - suggested that the Brits might have a chance.
But the smallest error was always likely to end British interest in a final packed with talent, and so Thomas and Purvis joined Orozco, Hambuchen and others in the list of casualties.
Uchimura, who struggled before qualifying for this final in ninth place, wasted no time in restoring his reputation as the sport's leading male protagonist by some distance.
The winner of the 2009, 2010 and 2011 World all-around titles briefly sparred with team-mate
who had been called into the final as a reserve for injured Japanese colleague Koji Yamamuro.
But a flawed Tanaka floor routine let in Uchimura, 23, for an Olympic gold medal that leaves his record in the men's sport unrivalled. Tanaka eventually placed sixth.
"Uchimura stepped up to the plate, he's a true champion. He fully deserves that," said Thomas.
Uchimura, who was not faultless himself, scored 92.690 for gold, ahead of Nguyen, who overtook
at the death for silver with a score of 91.031. Leyva of the United States took bronze on 90.698, while Thomas scored 89.406 and Purvis 88.332.
To put the pair's achievements at London 2012 into perspective, since Tysall in 1908, two 20th-place finishes are the best any British man has managed in this event.
Thomas still has the Olympic vault final to come and a chance to take one more swipe at an unlikely but, importantly, achievable medal.