I'm still baffled why, if I do nothing but keep it valid, my ICAO compliant PPL (the UK PPL) will be downgraded to an EASA LAPL with the associated limitations.
It won't be downgraded to an EASA LAPL. If you do nothing but keep it current it will remain a UK national PPL, which will not be valid for use in an aircraft with an EASA CofA. It might remain valid for use in UK-registered Annex II aircraft but
AFAIK the UK CAA have made no statement to this effect.
To fly an EASA aircraft, you will, not unreasonably, need an EASA licence. This may be obtained by exchanging your UK national licence for the JAA equivalent (which is deemed to be an EASA licence) prior to the enabling Regulation coming into force or by meeting the requirements of Part FCL for the issue of an EASA licence thereafter.