Thanks for all your informative replies.
Yeah, it's a bit of a shame that both the "National IR" from France and IMC in UK fall into the IR(R) category, but both of them are only valid in their own country ...
From what I gather, the full IR would be quite an investment in time and money (and even more so to keep it current), but would allow what I want, that is flight to France in reasonable weather without worrying about VMC/IMV conditions.
There also seem to exist a CBIR that I could potentially get after spending enough time flying with the IMC alone, but the information about it is scarce and I'm not sure it wouldn't be easier to just get the IR...