Wow, I clearly started something here! Thanks Beagle in particular for directing me back to the ANO. I think I've now found the relevant items - Article 153(1)(a) allows the CAA to include any rating listed in Sch 8/Part 2/Chs 1 or 2 in a UK PPL, and Article 163 allows a UK PPL to be flown on a medical declaration.
Needless to say, the pilot in question also has a LAPL with a LAPL medical certificate, so is unlikely ever to need to fall back on his PPL(SSEA) since he doesn't fly non-EASA aircraft.
What imaginative things might we all be doing if we weren't having to fill our heads with this mind-boggling arcanity!