I don't understand what you are describing.

Is it that a flight is showing F6 A0 etc, and a request to BA to release an A seat has been refused?

Or is it that the flight shows F6 A3, and yet BA will not allow a booking into A class with an AONE* ticket?

If it's the former, then I cannot see cause for complaint. If there are no A class seats available, you can request one, but it is a request. If it is not released, too bad. AA does exactly the same on the transcons and I would expect QF does the same on LAX/SFO-SYD/MEL.

If it's the latter, then I think you have legitimate cause for complaint, although you should be aware of the phenomenon of differential availability by region. Sometimes, what can be seen as available differs by territory. If that happens, you can either waitlist, or get on the phone to BA reservations in other countries.