having to fulfil the slightly higher medical requirements for a PART-21 licence declaration (the sub 5700kg requirements)
No you don't, the word "sub" is not used.
it means that any LAPL holder flying on a sub 2000kg PMD will have to cancel it
No they don't. The wording isn't sub 2000 kg or sub 5700 kg, the wording is:
Holders of UK national licenses only, to only fly aircraft no greater than 2000 kg MTOM. The wording is the same for the
"no greater than" 5700 kg aircraft.
The LAPL and NPPL privileges provide for flying no greater than 2000 kg MTOM aircraft. The self declaration for the no greater than 2000 kg aircraft is therefore all that is required.