I went to the game today and you're right. The problem is that these London matchups are basically exhibition games with no 'home' team - there were more people rooting for the Jets than the Falcons today, but they were both hugely outnumbered by neutrals desperate for any chance to see to a 'meaningful' game. Even if London were given a frachise it would take years to generate a home atmosphere as everyone here already has 'their' NFL team, which is one of the main reasons I don't think it will ever happen.
It's still a great atmosphere in terms of people having a good time, just coming together to celebrate the sport, though. I've been to several of these UK games over the years (I live about 60 miles south of London), and it's really noticeable how much more knowledgable the crowd is these days - they used to cheer randomly for no apparent reason, but at least the majority seem to follow the game a bit better now. The other thing that has increased with time is the number of Bills jerseys on show, which is great to see. It used to just be a handful of 40-somethings like me, veterans of the first wave of NFL popularity in England in the 1990's, but there were plenty of youngsters wearing their number 17's today. The stadium is pretty awesome; probably the best in the country, and it has no problem generating an atmosphere for the two soccer games which I've been at there. Certainly beats Wembley as the best venue for NFL over here.