The IR (r) qualification is also only valid on UK registered aircraft flying within UK airspace and has no validity on USA registered aircraft flying in any countries airspace including the UK.
FARS allows a foreign pilot to fly a US registered aircraft in the State that issues the pilots licence. There is no limitation of exercising any of the privileges held on that pilot's licence, including using an IMC rating. This was recently confirmed by the FAA's New York office in writing. It is of course not valid in another State.