Good points, TykeMan. I have my doubts, too, mostly based on the logistics of having one team located thousands of miles and an ocean away from the rest of the league. As for the level of support in the UK, Goodell seemed to say they're thinking of having games in Manchester and Glasgow in addition to London, and if they're able to get consistent support for those games, it would give them an indication of how much support they might get for a London team.
Given that London has over 7 million people, I think there would probably be enough support for an NFL team to justify a franchise. They'd only need to get 60 to 80 thousand people involved, and if you widened out to say 30 miles outside the city, I'd bet there are that many people who would support a team.
But like Karrit, I'd have to root against them when they played Dallas.