waterpau,
Perhaps, using the terms loosely, modular=part 61 and integrated = part 141. So when you say 'modular' I'm assuing you are thinking Part 61. So who said you need 150 hrs to start CPL training? Is this a recommendation from a flight school? Part 61 is a roll your own type affair, the formal training for the actual CPL might be around 20-30 hours dual, so you could in theory start this with 220 hours under your belt. Plenty of people do this in practice.
I've forgotten (or more likely never really understood) the immigration regulations. If you are a foreigner and going for a rating don't you need to be enrolled in a Part 141 school? Which brings me to ask, and it's none of my business of course, but what are you going to do with an FAA commercial ticket if you cannot work in the States?
Then there are regulations in the UK relating to training, and you will need to be signed off by a FAA CFI, so you need to find out about that. Could be another hoop to jump through.
There is one UK based designated examiner on the publically available .
FAA database. Ask him and see what he would need to sign you off. I still think you would need TSA authorisation even if you lived in the UK.
Having said that, if I was you I'd get an FAA PP-ASEL on the back of your JAA one and use that to fly around a bit in the States. Then when you are ready, and it shouldn't take long to get up to speed with the US way of doing things, just get your 'real' FAA PP-ASEL. Then do your IR, then CP-ASEL, then CP-AMEL with instrument add on. Should keep you busy