Not all that long ago, the ANO used to include restrictions on certain Class D airspace inside which mandatory compliance with IFR by IMC rating holders was not permitted.
For example, you could legally scrape over the cliffs into Cardiff VFR below a low cloudbase, but you couldn't fly an ILS in IMC.
This was later amended, so that only Class A airways are now off-limits to IMC rating holders; SVFR is permitted in Class A CTRs, but not in airways.
I am reliably informed by someone who has been digging around this a lot longer than I have that the words "UK only" were also missing from the ANO, not many years ago, on the IMC Rating IFR privileges
The bizzare thing is that the CAA was free to put out an ICAO Difference on the IMCR. Had they done so, it woul dhave been valid worldwide except where the airspace owner objects.
my understanding is that the FAA views the FAA PPL/IR as equivalent to the UK IMC rating in G-reg aeroplanes.
Not heard that one anywhere. The FAA told me, in writing, that they regard the IMCR as equivalent to the FAA IR (within the limitations of the IMCR itself of course i.e. UK only, no Class A etc).
The privileges in a G-reg are up to the CAA, not the FAA. As regards how the CAA views the FAA IR, to the extent that for £70 or so an ICAO IR holder can buy himself an IMCR. In the case of the FAA IR he needs to be within his IPC (i.e. the CAA disregard the FAA rolling currency rule). He also needs a UK/JAA PPL or CPL, a UK medical, etc. It's not a terribly useful maneuver but a G-reg pilot gets IFR privileges for Class D, whereas with an FAA IR he would have them only in Class F-G (which is close to useless for going places).