From what I understand Flight Saftey in Farnborough do the written exam. Your hours in the UK will count, 250 needed for FAA CPL. You will also need an FAA class 2 medical, which can be done in the uk see
www.faa.gov for the list.
A good starting point for you will be FAR/AIM 2007 (Federal Aviation Regulations/Airmans Information Manual).
There is an instructor at Western Air in Thruxton that can do a lot of FAA ratings. His name is Peter Metcalff see
www.westernairthruxton.co.uk
Hope this helps.