A LINE OF COUNTERARGUMENT
For some reason your generosity in setting out the counterargument reminded me of the line about the monks who have spent so much time together and have such a limited repertoire of jokes that all one of them needs to do is say the joke
number to get the rolling in the aisles...
Perhaps we should have a numbering scheme to save typing?
You can no more pick and choose which of the licencing FARs you go along with, than you can elect to maintain it in accordance with UK maintenance schedules.
But the point is that UK law specifically delegates that to the country of registration:
Art 8(1) Subject to paragraph (2) an aircraft shall not fly unless there is in force in respect
thereof a certificate of airworthiness duly issued or rendered valid under the law of
the country in which the aircraft is registered or the State of the operator, and any
conditions subject to which the certificate was issued or rendered valid are complied
with.
For a UK citizen, the important aspect is that if you don't maintain the aircraft according to FAA requirements, you're in breach of Article 8 of the ANO.
Art 14 similarly delegates the equipment requirements. Art 76 delegates the requirement for carriage of documents. And so on...
That sort of delegation is required for compliance with the Chicago Convention.
But Art 21(3) is slightly different. It says that you must have a licence granted or rendered valid under the law of the state of registry
or under the ANO.
That's rather different. It's making explict provision for a UK licence which is
not granted or rendered valid by the state of registry to be used. If you're suggesting the aircraft must be flown in accordance with the law of the state of registry in every respect, including their personnel licensing rules, what possible reason would there be for providing an alternative?