You've got to the heart of it, really. In these modern times air crew have the ability to act with impunity and it doesn't matter how in the wrong they are, you can end up in quite serious trouble if you decide to stand your ground.
I fear that this long after the event - you'll be on a hiding to nothing. Time has passed, the crew and BA will question the veracity of your recollection. Complaining will also not get you anything in the way of knowing if the crew concerned were warned about their conduct for reasons of confidentiality.
In your shoes, I'd be taking another way of complaining open to you - and that would be taking your business to another airline. It's meaningless in the general scheme of things, but perhaps it'll give you some closure and fully-flex tickets in Y to Sydney are not cheap.