I agree with University of East Anglia about visiting all four medical schools as 5-6 years is a reasonably long time to spend in one place. I think it's probably also true that the majority of medics choose to settle in the region in which they studied. There is absolutely no reason why you should do that (applications for the next stage are blind to which medical school you attended) but 5-6 years is long enough to start putting down roots (friends, partners, etc). Your personal finances might also play a role as, although London is an excellent place to experience, it is much more expensive than anywhere else.
The good news is that, whichever course you choose, you will almost certainly think it was the right choice when you look back in 10 years time.
Congratulations on four offers!