From what I understand our friends at Gatwick will issue you an FAA Med at the same time as a JAA one. However if you already have your JAA medical I think you will have to sit the FAA exam.
You can get a list of all the FAA AME from the FAA website
http://www.faa.gov/pilots/amelocator/
Remember that you will need form 8500-8, again you can do this online at
https://medxpress.faa.gov/
For FAA medicals there is no initial like ours so all the examinations are to the same standard. You only need a class 1 for ATP, class 2 for CPL and class 3 for PPL. In fact there is little difference between the examinations, but class 1 does have higher eyesight requirements and ECG, expect a cost of about £150.
Hope this helps