I'm soon off to the States on a J-1 visa to obtain commercial and instructor ratings and build hours as a CFII/MEI. What you must remember is that you will earn FAA ratings which will need to be converted to JAA at great effort in terms of time and expense. I already have an FAA PPL and JAA ATPL theory, so although the conversion will be expensive at least it will be fast. I reckon that after all training and living expenses, by the time I reach 1000hrs and JAA FATPL I'll have spent £35000-40000, so no huge saving over training in Britain. There is no way a school can guarantee a job for you. This depends on your ability, student load and instructors moving to regional airlines. Pay will also be very poor and will probably not cover living expenses. Obtaining the J-1 visa is also a minor pain in the ass and will involve a trip to the US Embassy in London. On the up side, you'll be able to instruct for multi, IR, CPL and CFI ratings; not possible in the UK with low hours. Your hours would build far quicker than in Britain, which should increase your employability faster.
So, the choice is yours J1mbo. Why not obtain your PPL in the States to see how you like flying and get the ATPL theory out of the way first. This will make things easier in the long run.