Having just completed a CPL ME IR in the US, you need 250 hrs for the CPL of which 50 can be in a sim or flight training device. I haven't much experience of JAA as I can work over here. It took me 8 months to go from zero to CPL ME IR, I am starting a CFI course now, but after that who knows whats going to happen. If you have the hours and the required aeronautical experience (40 hrs total, 5 hrs x-country solo, 3 hrs dual at night inc 10 takeofs and landings, plus a few other things)a UK PPL holder should be able to take the FAA PPL test reasonably quickly, the knowledge test is easy and the you can even take it in the UK at Gloucester Airport. I imagine the practical test is similar to CAA/JAA, cross-country portion, diversion, stalls, slow flight, steep turns, ground reference maneuvers, engine out approach, hood work, a bit of VOR work, then back for landings - soft field/shortfield , slips etc. I'd be glad to help out with any specific questions.
It looks bad but you have to remain optimistic!