What is quite clear from the latest NPA is that in order to obtain "grandfather rights" the holder of the rating will have to meet the JAA IR standard
No, quite the opposite is clear to me from the NPA. An IMCr holder will need to have an IMCr (probably a valid one) in order to be issued a restricted IR granting IMCr privileges in UK airspace.
The content of the IR is all encompassing, and so it should be if you want to sit in front of 300 paying passengers or more, 6 miles skyward, hurtling along at 500mph, without looking where you are going
The IR has no content specific to flying jets/6miles up/500mph/300pax any more than the IMCr has. You can be trained and tested for the IR on a Cessna 150.
It seems to me that if the French can just pull an IR out of thin air just because they wish to (apparently without endless months of committees, debates, meetings etc...)and seemingly get no complaints from Europe
That's because the new French IR is not an EASA FCL qualification, just as the IMCr isn't. They won't be valid under EASA FCL and will need a conversion deal also. The UK could also pull such an IR out of thin air too, at present, because EASA FCL is still not in force and won't be until next spring with the variations transition periods following.