One legal question: If (for arguments sake) Unite win at the High Court, is BA actually required to revert back to the old crewing levels, or is it just required to pay damages for breach of contract (which on an individual crew level must be tiny)?
As our common law system works on the basis of damages, I think it may be later.
This post is not legal advice.
English law recognises two sets of remedies for dispute: the 'common law' remedy of damages, and a variety of additional 'equitable' remedies, such as injunctions.
Damages are available to a successful claimaint by right, so if the crew suing BA prevail they will be able to claim for what they have (individually) lost as a result of BA's imposition. Given the complexity of the agreements this will be hard to calculate!
Secondly, they may in addition be given an injunction requiring BA to return to the arrangements before imposition, provided they meet a number of complex criteria. All remedies apart from damages are at the discretion of the court, so they cannot be guaranteed. Most litigants obtain what they want without problems, but were there (for instance) evidence that crew were being unreasonable in their reaction to the imposition, they might be held not to have 'clean hands' and an injunction would be unavailable. That said, even if no injunction was granted, if BA continued to break the crew's contracts BA would be liable for damages for every further day the contracts were broken, so, were the crew to succeed, it would be unlikely that BA could avoid returning to the agreements in the short term. [This could be difficult if BA have shed so many staff they haven't enough to return to previous crewing levels!].
Moreover, a ruling that imposing terms broke the contracts would not be an absolute bar to BA seeking to modify the contracts, provided it did so lawfully.