I've read a few times, that they have actually looked into that, having a full time team in the UK that is.
Two things. 1. The distance between London and New York is roughly 3,400 miles and the distance from New England to San Francisco is roughly 2,600 miles. Their argument being, that NFL teams already travel big distances for some matches, so a team stationed full time in the UK, isn't that much of a stretch in that respect (Kick Off times aside)
And 2. The NFL has been wanting to expand the game beyond America for some time. Its a bit hard to call yourself 'World Champions' when the only country you play in, is the US.
So it wouldn't surprise me, if at some point, they do station a full time team here.