Dorking - Agreed the legal situation is a minefield.
In fact mask wearing isn't that simple either. On the ground in the UK - not enforceable but outside UK airspace that could change.
Too complex for me but this article appears to cover it quite well if anyone's interested. I should add, I am not a lawyer so can't guarantee it's accuracy but from memory it sounds about right.
Which Country's Laws Are Enforced During International Flights? - Simple Flying