A quick check of the ANO shows that is not legal to use the IMC rating outside of the UK. It specifically states it is only valid when in the UK. There is not even a mention of the Channel Islands or IoM, but the CAA's skills test guidance notes for the IMCr state "It is only valid for flight in UK territorial airspace, the Channel Islands and Isle of Man airspace".
So even if the French say it is valid, the UK ANO does not. You have to comply with the more restrictive regulations, which means no IMCr outside of the UK, IoM and CI.