The privileges of an FAA IR RATING need an N-reg plane, anywhere in the world.
With an FAA PPL LICENSE you can fly a G-reg (UK only) or an N-reg (worldwide).
With a UK/JAR PPL LICENSE you can fly an N-reg (UK only) or a G-reg (worldwide).
If you have an FAA IR and need to fly real IFR in the UK, you can get the IMC Rating just by sending off a cheque to the CAA. But an IMC Rating also needs a UK/JAR PPL.
A UK/JAR PPL needs the Class 2 medical. An FAA PPL needs the FAA Class 3 medical.
The above is AIUI (as I understand it).
I have heard stories of pilots with an FAA IR getting permission to fly IFR in e.g. France in a non-N-reg plane. I don't know these first-hand but I heard of it in connection with aircraft delivery flights.