I think it should be optional.
There is no legal requirement for registration of U.K. yachts in U.K. waters, and even if on SSR there is no port of registry.
However, as with some forms in overseas harbours, putting in a “home base” harbour, if you feel you have one, is sometimes simpler than having the debate
Port of Registry is probably just a legal term being used, I think Ofcom can only issue a licence for boats registered or kept in the UK, Northern Ireland, Chanel Islands and the Isle of Man, so as long as the port you keep the boat in or it is on a British register they can issue a licence, if your boat was kept in Amsterdam or elsewhere you would go to the authority of that Country. So for port of registration, where you keep it is the Nswer to port of registry.
No doubt someone will now say I ‘m wrong but that’s life.