What some clever folks do is do the JAA ground exams in the UK, then go and do the FAA PPL. As soon as the FAA PPL is in their hand (2 minutes after the flight test) they then arrange to have a test the next day with a JAA examiner.
Although a school needs to be CAA approved to train for the JAA PPL, a CAA examiner DOES NOT need to examine from an approved school to carry out LICENSE CONVERSIONS.....
That is really clever. But doesn't it require the FAA PPL holder to have 100hrs TT?