Check out
www.caa.co.uk/srg for information direct from the CAA.
I believe it's similar to what I discovered for a f/w ATPL. One can swap a non-JAR ICAO ATPL for a UK ATPL with just 3 writtens but then that's only valid on UK-registered aircraft. For JAR validity one has to do all 14 tests.
I have a Nigerian ATPL (Airplane) so that was what I was asking about. I don't think the FAA license does much for you, although I did once get a German CPL-IFR (Airplane) on the strength of one.
Another snag is that one must do an initial Class 1 medical with the CAA at Aviation House, Gatwick Airport, London. That costs about 275 pounds, including a preliminary eye examination with a civilian optometrist. One must pre-pay when booking, by credit card. At least there is a free bus service from the South Terminal of Gatwick Airport that saves you a taxi fare.
Since everyone got on this JAR bandwagon things seem to have really tightened up. Good luck!
Check out Naples Air Center for US-based training. They have an advert on the training part of this website, actually.