You might want to check out the thread "Med School in the UK." I asked a ton of questions about the possibility of people from North America going to school over there and got lots of helpful suggestions and insight into the whole process.
Anyway, according to the British Council, the following schools accept North Americans: Birmingham; Bristol; Leeds; Leicester; Guy's, King's, and St. Thomas's; Newcastle; Nottingham; Oxford; Sheffield; Belfast; and Dundee.
Now I contacted all of these (and even some of the other ones that supposedly don't consider North Americans), and I noticed some discrepancies.
1. Leeds told me I wasn't eligible to apply to their medical program. (This could have been related to the fact that I've got three strikes against me: I'm an overseas applicant who'll be 28 at the time of matriculation and whose degree is in English, so I'm currently going back to get my science prerequisites. For whatever reason though, they did say no.)
2. Southampton, which according to the British Council does not accept North Americans, actually sent me a packet of information about their school, and a Southampton student said that there are Americans in his med school class.
3. Cambridge is not on the list either, but there's a Canadian student on this message board who has been accepted there.
As for which schools particularly favor North Americans, if friendliness and helpfulness from admissions tutors is any gauge of that, I'd have to say that Oxford seems very encouraging, with Sheffield coming in second. The people in their offices have truly gone out of their way to look into my situation and help me out. The people from the other offices were nice as well though. As for Bristol, they have yet to answer me, so I don't know.
If I were you, I'd go to
www.bma.org.uk. There you'll find a list of British med school Web addresses and email addresses. It's probably best for you to check with the schools individually. Good luck!