Aussie Andy
If you have a FAA IR then you get IR privileges in the UK outside controlled airspace.
If you then write off to the CAA for the IMC Rating, you get the full UK IMC Rating and all its normal privileges, no restrictions just because it was based on the FAA IR (
AFAIK!)
The best way of course, for an FAA PPL/IR, is to fly an N-reg plane and then you can go anywhere!! Plus you get loads of other benefits, e.g. own maintenance, no star annuals, can fit various non-CAA-approved STC mods... much easier (Class 3) medicals