Johnny, Fatflyer is correct - they will issue you the UK IMC on theback of your FAA Ir, after paying the £65 of course
The FAA IR costs $3300(in CA), the big advantage is that you get two licences for the price of one and also the FAA IR is a 40Hr course so you end up with a lots more hours for your money!
You may of course want to take the FAA IR if you intend to hire an N Reg over here - unfortunately you have to track them down as they can be hard to find!
If you want to take the IR exams I can reccomend American Aviation based at Norwich Airport, 01603-301902. It was a 2hr drive for me but more than made up for it with the quality of the school (1 on 1), very reasonable fees (£75) and Tom Houston is a genuine helpful, nice guy!
Julian.