Several things from my own experience:
Consider EHC in Norway for an ME IR(H) - excellent school, and they compare very favourably to UK costs (0% VAT!). I believe you can now train there in an AS355 as well as the original Bo105. Accomodation on site, good selection of approaches at home base, and lots of budget airlines flying direct to their airport.
The CAA will require a copy of every single JAA approval (school, instructor, examiner, aircraft, etc. etc.) from a non-UK school, but assuming you have all the paperwork, it's no problem getting them to add the right things to your license (what? The school you chose was JAA-approved, right?). I assume this will continue, replacing JAA with EASA.
Of the 3 major offshore companies in the UK, only Bristow will accept a SE IR; CHC and Bond both require a ME IR to even consider you. Bristow, however, require a minimum of 500 hours ME time, if you didn't attend Bristow Academy, so you may not benefit from the SE IR anyway. The above applies to low-experience co-pilot applicants, obviously - if you have a bunch of useful experience, they'll probably make an exception on the SE IR rule.