A few things that lead me to think the IMCr is not valid outside of the UK, IoM and CI:
- CAP804 (LASORS' replacement) is not a legal document.
- The ANO, which is law and overrides anything CAP804 says, doesn't mention about being able to fly in another in country with the permission of the national authority. You can be prosecuted for a breach of the ANO - not for CAP804.
- The ANO specifies the privileges of a PPL - including minimum flight vis of 3km, non-IMC etc. These apply worldwide (unless there are more restrictive regulations in the country you're flying). If an IMCr privileges are only "within the UK" then the PPL privileges apply elsewhere.
- The CAA's skills test guidance for the IMCr states "It is only valid for flight in UK territorial airspace, the Channel Islands and Isle of Man airspace.". This document has as much legal standing as CAP804.
Regarding the ANO, I said it allows the IMCr privileges "within the United Kingdom" (that is how it is worded in the ANO). I know it doesn't specifically forbid the IMCr outside of the UK, but it doesn't permit it either.
I suspect that "The IMC Rating may not be used in the airspace of any other Country unless permission to do so has been given by the appropriate authority of that Country" is for the benefit of the IoM and Channel Islands.
If only the CAA could make things clearer there would be no need for these discussions!