In contravention of UK law which the pilot of a G Reg has to comply with.
But probably legal if it was an F-reg. Then it's the DGAC which sets all the rules, including the privileges and restrictions that apply to the validation of your UK-issued PPL by the DGAC. (At least, pre-EASA.)
But I agree with the sentiment here: I'd like to see the exact wording of the DGAC (in French if you don't mind) regarding what exactly was accepted.
I suspect that all the DGAC accepted was the fact that the IMC lifted the VFR-on-top restriction, but not that the IMC would give the privilege of flying in IMC conditions, OCAS, in France.