NPPL(M)-->NPPL(SSEA) needs to be done by an RTF or ATO, followed by a skill test.
NPPL(SSEA)-->NPPL(M) can be done by any suitably qualified instructor, but a skill test is also then required.
Either can be done on an LAA aircraft so long as the pilot being trained is an owner or part owner (or it's a type approved microlight).
And RTF / ATO needs to go through some process to add the aeroplane onto their books, but I believe that's reasonably straightforward.
G