I'm wondering this too. The London Expat American Meetup Group is doing something, but I'm not sure I want to go. The organiser is kind of weird...apparently he sets up some of the meetups so he can make a profit. This one is free (I think), but not necessarily sure I agree with some of his other (possible) tactics. Then again, I've never met the guy so who knows? The event is there, in any case. They don't tell where it is until the day, for fear of others "breaking up the group" (??).
There's something else happening at the Benjamin Franklin House, but looks like it's sold out. And the Democrats Abroad had their annual Independence Day picnic last Sunday (boo).
I'm sure you can go to like, Bodean's or Blues Kitchen in Camden and get all American...think most of the American themed restaurants and bars are doing something or other. Not ideal though.
Crazy that it seems this is the only thing going on!
http://www.meetup.com/americansabroad/events/51140502/