Lurker's argument that 'because the contract refers to collective agreements as part of the contract, and crew complements are part of that agreement, therefore are contractual' is rather simplistic and there are other considerations that are relevant.
(There have to be, if you think about it. BA spend huge sums on employing some of the best legal brains, and these guys will have looked at every proposition and confirmed its legality in far more depth than we legal amateurs on a forum).
I think the weakness in Lurker's assertion is that English law, and employment law in particular, despite what might be written in stark black and white, is subject to the principle of 'reasonableness'.
Even though both parties may have signed a document saying x is always going to happen, if one party later feels that x is 'unreasonable', especially if circumstances have changed since the document was signed, then that party can challenge the contractual statement and say it is not reasonable. For example, dress regulations might form part of a contract of employment, and if someone came to work with their hair 1cm too long they could be sacked. Any employment tribunal would say that although the letter of the contract was followed precisely and literally, the interpretation was 'unreasonable' and the sacking would be deemed unjust.
BA will, amongst other things I'm sure, say that although there are many hundreds of points agreed in the official 'agreements' between the union and the company, they cannot 'reasonably' be required to renegotiate with the union on every item. They will also point out that any crewmember in excess of the minimum stipulated by EASA CS25 is purely for commercial reasons and BA must 'reasonably' be allowed to make commercial decisions.
Especially if they can show they have tried to negotiate, but the other party has refused outright to discuss the issue (see Bassa comms), they will feel that they have a strong argument in court.
There are other legal areas which grey out the black and white of lurker's argument and which BA have doubtless prepared in depth, particularly following the judge's statement that he wasn't 'minded' to grant Unite's injunction, which was purely an argument of whether the impositions were contractual or not
In any case, I'm pretty sure it won't get that far. Once Unite set a strike date, the whole tone of the argument will change. BA will go on the attack with far more artillery than the BF softly softly 'trust me' argument. It will get very nasty and the euphoria of week Monday's ballot result won't last very long at all. We're still in the phoney war, where the yes voters think their ballot will force WW to back down. The trouble is, WW knows the vast majority of cabin crew won't strike. He's absolutely convinced of it.