I am so excited about this! I've been a Vikings fan since I first got into American Football at university and the chance to see my team play live is an absolute dream come true. On the other hand, I know how much it must suck for fans in Minny to lose a home game, and I do feel bad about that.
There is a far bigger fanbase for the NFL over here than you might think (and has been since Channel 4 first started broadcasting games in the 80's). Granted it'll never be one of the UK's 'main' sports but it has a good sized following. I also think the International Series games do have an effect on growing that support - one of my workmates came along to the Niners/Broncos game a few years back out of interest and he's since gone from being someone with almost no knowledge of the game to a proper fan (49ers).
There are a number of amateur British teams as well as a good few university teams. I even had a go at playing Cornerback for one myself (Antoine Winfield I was not :D). But then I first laid my hands on a football when I was already 20, there are now youth team programmes starting up that will hopefully get people playing the game earlier and developing their skills. As part of that, being able to see top-flight football being played live does wonders for wanting to play the game yourself.
Either way, I'll be there next September - doing my best to keep Vikings homefield advantage alive in London. ;)