Thanks for the replies so far. I hold a UK PPL and various other flying quals but need an FAA PPL in order to do the recreational flying I'm planning (I do not have time to wait for the 'verification of authenticity of foreign license' process).
You do NOT have time for the verification of authenticity of foreign license process, but you do have time to go through the TSA and visa hoops, plus all the training (including night x-country, skills test etc) for an FAA standalone PPL?
When I did this to go to Florida for my initial training, getting an appointment at the embassy for a visa interview took six weeks!
I know what I'd do if I were in a hurry. Go to Gatwick, have the CAA issue the validation document, Fedex all the required documents in advance to the FDSO of my choice. Upon arrival, visit the FDSO, get paperwork issued, then get a BFR and off you go. I believe for a piggyback FAA PPL the only medical you need is a JAA medical to make your JAA PPL valid. It sounds like something that can be completed in less than a week, particularly if you present yourself at Gatwick in person, instead of doing things by mail. (And on a side note, contacttower recently flew into Gatwick in a light single for a CAA appointment. His story is in another thread.)