Try a place called The Sports Cafe, its relatively close to Picadilly and that whole area. I was in London Week 1 and had to watch the Jax game there. You have to go there late night obviously due to the time difference, but as soon as you walk in two things will help you feel at home.
1) baseball caps everywhere, that's a sure sign of Americans.
2) Guys will no style whatsoever, again another sign of Americans.
They have a bunch of TVs, and like I said, you can meet some fellow fans from pretty much any team. I met some kids from whatever school Southwest Texas turned into (the one in San Marcos) who were abroad on a study program. One of the girls was real cute
and grew up near me, so we met up on another night. Its like the only place in London I know of. Check this article for this guy's opinion. good luck