AIUI the way pilot licensing works, worldwide, is that your privileges are valid worldwide unless otherwise limited, but they can also be limited by the country in whose airspace you are flying in, by the aircraft equipment etc.
Any license restrictions also apply worldwide.
So the CAA could have made the IMCR valid anywhere, but it would not have done them any good because nobody else recognises it (as a whole)....
The apparent ability for a PPL+IMCR UK pilot to fly above the layer in France is very useful nevertheless.