So an FAA licensed pilot in a G reg can fly worldwide because his license is issued by an ICAO state. BUT he can't excersie the privelidges of his IR in controlled airspace in the UK......what about IFR in other countries.
The Rule that allows you to do it, and places the limitations on its use (Art 26) relate to the operation of the aircraft and not where its flying, so you can't fly in controlled airspace in IFR anywhere if exercising the privileges of an ICAO licence.