hello
If you have never had any JAA medical, you'll need to pass the JAA initial class I. the only place as far as I know in the UK is in the Gatwick medical center. The cost is 330£. It takes half a day to be completed. If you have had already a JAA class I , and you're looking to renew it or revalidate it, you may find many AMC's around london area. check out this link:
Medical | Safety Regulation
AME Search Results | Medical | Safety Regulation
Good luck