It is going to cost an absolute fortune. Don't know much about helicopter licenses but I think you need to do the IR in a twin which means turbine and loads of cash. You are in for a shock if this is the first research you have done. Try searching on here or the CAA website or call a school. CAA is the best place.
Found this from a post in 2006 on pprune so probably more expensive now for the IR:
Specialist Aviation Services - IR from scratch is about £40K (+VAT) inc ME type rating, but for FAA->JAA conversion including (for example) AS355 type conversion someone here was quoted £15K +VAT.