On the news this morning they were saying it should be Bradley Wiggins, which would be fair, but it's apparently already been decided (but won't be revealed until the ceremony itself).
Really, I think it should be David Beckham. He's the closest thing Britain has got to a global sporting megastar, he's always made his patriotism clear, promoted Great Britain around the world for well over the last decade, and it would be a nice geture given all the work he put into winning the bid/promoting the Games only to be snubbed from a place in the Team GB soccer team.
Failing that, Steve Redgrave would be a good choice as Britain's Greatest Living Olympian.
My real concern is that it'll be some fucking underprivileged hippidy hop dance troupe from Hackney doing a shitty routine from Britain's Got Talent. Or Will.I.Am.