Well if you have work rights for America and are contemplating living there, and it works out cheaper, it would seem the most logical route to take.
The FAA training can be conducted in the UK or America - it depends on what suits your circumstance.
On the theory side of things, there is 1 exam for PPL, and 1 exam for CPL. They are multiple choice and can be sat in one or two locations in the UK. If you buy the correct literature you can even see all the questions that are in the database.
Flying-wise, the last 3 hours must be signed off in your log book by an FAA CFI. There are no end of CFIs floating around the UK who did training in the States before returning. Your flight test (and oral) can be conducted by either an FAA representative when he/she is in the country, or I believe there is a resident FAA examiner in the UK although his name has slipped my memory at the moment. For some reason Coventry and Hull ring a bell.
As for the CAA: If you have a valid ICAO CPL(H), you get a reduced Class 1 medical (both in duration and cost, thankfully) and also the CPL training is as required, determined by Head of Training at the school you choose. The stumbling block is the theory, and what your exact requirements are going to be would be best explained by the CAA themselves at SRG.