That's why I would not choose the altitude limit arbitrarily, but connect it to something that, in practice, is a limit already. Without oxygen/pressurization you cannot fly above a certain altitude (except for a short while in FAA-land) and without RVSM capability in the plane the upper flight levels are forbidden anyway. So those would make natural limits for an IR-light, since you don't have to explain a lot more to the controller if you get such a clearance.
I'm just looking at my club environment where we have a bunch of stock standard PA28s, 172s and a DA-40. Most of them IFR equipped and certified. That's the kind of airplane that a lot of PPLs would want to do their IR-light in. And that's probably what they're using for their IMC today in the UK in any case. They rent, not buy. And as a result of which, even if the airplane has the capability to cruise above the oxygen level, there's no club which rents oxygen cylinders, masks, regulators and stuff along with the plane.
We are primarily looking at a pan-European replacement for the IMC so I think you've got to look at what the types of airplanes are, and the experience levels of the pilots who use the IMC.
And as I said, if you want more capability, get the full IR.