Plus the weather is bad in London in Jan/Feb, and it's still midst of soccer season over there. I really think the attempt to globalize the game is ill-conceived. It's one thing to play an exhibition game, but taking a regular season game away from the fans here is an insult. At the moment, the strategy seems to work because the game sells well because of the novelty value, and because Brits lap up anything with an American label on it. But if the talk of locating a team there is serious, I think that's a huge misstep that has many ramifications, not the least of which is logistics.
I get the feeling you would see a full time team in London before a Superbowl over there. Either way, taking the Superbowl out of the states will cause a HUGE backlash, and for a league obsessed with image, that's likely something of a deterrent.