... unfortunately this does not apply for conversion from UK national to UK Part-FCL.
It shouldn't be a conversion. A UK PPL is the same as a EASA PPL or even this particularly factious Part-21 PPL. All that needs changing is to an up to date piece of paper, IT IS NOT a conversion. There is nothing in the training nor in the privileges of the licence which is different. The PPL A licence holder met all the requirements for ICAO and still does. The restriction to UK airspace only was artificial and to placate the Czars of the EU. This unfair act simply needs reversing and then we're done.
A Part-21 aeroplane is just the same as a Group A aeroplane. Of course things evolve and change over time, both for aeroplane approvals and the maintenance and therefore the pilots training, but that is normal and it doesn't need a change in licence.