Ebbie - what whopity says above is correct. You need to fill out form 2140 and have an Examiner sign it to say that your knowldge of UK air law and procedures is satisfactory. You can use your FAA medical. Thats all you need. Here is the link to the UK CAA website that will give you the full info
SRG2140: Declaration for Pilots holding an ICAO Compliant Flight Crew Licence Issued by the FAA for use in UK Airspace