If you hold the IMC, you cannot use it to fly IFR outside the UK, you're limited to VFR.
Quite true but this is because the laws of, for example, France require an Instrument Rating for IFR flight and not because of anything in UK law.
The issue I was raising was not IFR flight abroad, which I think is quite clear cut. Where it gets tricky is that French PPLs can fly out of sight of the surface under VFR whereas UK PPLs cannot, even in France, because they are bound by the rules layed down in the UK ANO. However, if the UK PPL has an IMC rating, that prohibition in the UK ANO appears not to apply. The same goes for the 10km vis requirement for SVFR flight.
GASIL seems to suggest that UK law forbids a PPL+IMC flying out of sight of the surface when abroad. I don't think it does but I could do with a lawyer's opinion!
Mark