The Mountain Rating is actually a restriction of your privileges. The IMC rating is an extension of your privileges. Do you see the difference?
To clarify, what I think Brendan is saying is that a UK PPL does not contain any restriction on landing on high altitude airports, whereas the IMC rating adds the ability to fly in conditions not permitted by the PPL.
To give a concrete example, I was entitled to fly VFR to Big Bear in California (6700amsl) as PIC, on the basis of my converted UK licence, but I could not to fly in IMC as my license privileges did not include that and I didn't hold an IR.
Obviously I took specific training to tackle the challenges arising from the mountain flying involved to get to to Big Bear, but no rating was mandatory to act as commander.
Seems pretty clear.
Having looked at photos of a French altiport, I completely understand why the DGA requires a rating for these operations, but nonetheless, this does not affect Brendan's logic.