Rossco..
Whilst no conversion should be needed when you return to the UK - you must plan on flying a few hours with an instructor at your local airfield anyway.
The licence needs no conversion but...YOU DO!!
You have read it many times before - procedures are different in the USA than they are in UK, the rules of the air are different, aircraft performance is different...etc. etc.
Also, between Naples Air Traffic Control and your flying instructor you will be taught how to find "the Courthouse", "Gordon's Pass" and "the toll booths"......and how to get from any one of them to the airfield. None of this will mean anything to you when you get back to the UK.
How long are you going out there for?? If you want a really good kick start to your training do as many written (and R/T practical??) exams as possible before leaving UK soil - also highly recommend getting JAA medical (class 1 if you intend to go professional, class 2 if staying as a private pilot) BEFORE going out to the USA. Indeed, with a bit of homework, you should be able to find a medical examiner in the UK that can issue you with BOTH the JAA medical (required for your licence application) and an FAA medical (required before you can fly "solo" in U.S. airspace - part of your course). If you do find an FAA medic, ensure he/she also gives you a student pilot certificate with that medical.
Last recommendation - get a visa before going. Another Florida school had a student turned back at Orlando recently....after somebody reported to the Immigration officials that [the student] was reading a flight training book (Trevor Thom) on the flight.
Above all else...have a marvellous time.