If I understand it correctly,
a UK PPL does not contain any restriction on landing on high altitude airports
but
the DGA requires a rating for these operations
so you just have to know that there's a restriction not actually written in your licence.
Whereas the UK PPL doesn't include IMC, so you know you can't fly IMC without an additional rating. Well that's a wonderful bureaucratic nicety.
So if we just didn't say that the basic PPL didn't include IMC capability (but put it in the rules somewhere) then it would be OK?
You're kidding me, aren't you! This is Sir Humphrey at his finest.
There's no F in Great Britain, but there is in France; that's another difference. That's another good reason why an Altiport rating works in France but the IMC is no good in Britain.