If you can get an employer to sponsor you, the UK CAA may give you an exemption to allow you to fly on your FAA for up to one year. After that you will need a JAA licence - CPL/ATPL, as appropriate, a JAA medical - which will probably require you to go to LGW for the full initial, though am not certain about that - and then you will need to get a JAA type rating.
As far as getting a full JAA CPL/ATPL in concerned, I don't think you get any exam exemptions, either for the JAA licence theory or any of the type technical exams appropriate to type. Your logged hours will be taken at par, as long as you haven't logged any PIC time when someone else has done the same on the same flight (FAA used to allow this, but I don't think they do anymore)
Best bet is to email the UK CAA direct and ask what is involved in licence validation/conversion, but am pretty sure what I have written up here was correct a year or two ago......
Of course, you would also need to have the right to work in the EU, or get sponsored by an operator.
f&r
(UK, US & Can ATP)