Just to confirm it was a poilte way of me telling ATC I was not going to do an orbit, as commander of a public transport aircraft then and now
Before we start arguing about the semantics of llanfairpg refusal to orbit on final, let's all agree that this was a highly unusual request at a highly inconvenient time. I'm sure all of us have had one of those one way or another, and did not have time to think of something to say that would be really appropriate, to the point and according to CAP413, but you've got to say something, not?
If llanfairpg had his brains wired in "public transport" mode (on time, on budget) and was already concerned about his landing weight, I can understand why he, at the spur of the moment, decided to add that information to his refusal, without thinking completely through whether those were indeed valid and legal arguments to use against a controller who, arguably, had his priorities wrong.
In any case, does a controller know, or is he/she supposed to know, which Aztec on final is a public transport flight, and which Aztec is close to max landing weight? Other than maybe a public transport flight callsign instead of the fuselage callsign, is there anything in the flight plan or otherwise that informs a controller of such? So the whole idea of saying "Confirm you know that..." followed by information that the controller has no way of knowing is already a bit odd.
In the spur of the moment we all say strange things. But the main message is: you can refuse such requests, and you should if it threatens your flight safety.