I'm going to King's College in London for a month in April--they're the largest medical education institution in Britain, so I might start looking there. That said, you should check with your med school's international med office to see if they have reciprocity with any schools in the UK, which makes the process *much* easier.
King's College London is the best bet for most students I'd think, as they're accustomed to accepting a large number of international med students. I'd start working on it soon, though--it took me the better part of last year to make all of the arrangements, and the whole process was very involved (the application, fees, immunization forms, finding accomodations, etc.). Also, KCL's application deadline for rotations during 2005-06 is early (April 1st). That said, the staff is really helpful and they've got every specialty rotation under the sun to offer. Their medical elective website is: