Just bear in mind the "Additional IR training as considered necessary by the Head of Training of an approved FTO."
Pick your training provider in the US carefully (on quality not cost) - I have heard of some horror stories of people returning only to have to undertake almost an entire IR course to get up to speed, thus making it more expensive than having just done the full IR in the UK.
Call up the 6 (soon to be 7, I hear) providers in the UK and ask them to quote for conversion, and ask what their previous experience on such conversions is