"In other words, it's in BA's interest to have an unprotected strike and mass-produce P45s? Interesting hard-ball game."
BA have lost a huge amount of revenue over the last 14 months. So yes, I think an unprotected strike is in BAs interest. They wouldn't have to sack everyone. Just those they wanted to. An ongoing strike over the summer that never ends and continues to hit bookings isn't in BAs interest. But ironically, the strongest suit Unite now have is over the threat to strike hitting advance bookings rather than any great ability to seriously hit actual schedules.
"Meanwhile, suing the pants off Unite for ... what exactly? Having a legal strike which cost BA a lot of money? I'm losing the plot here, somehow."
The strike was never ruled legal or illegal. An injunction to stop the strike failed (despite the union not meeting the requirements of the law) but that doesn't stop any future action, especially if new information has come to light. It would make a lot of sense to sue Unite for the £125 million that BA openly said they lost. It would also open the doors for "legimate" sackings of hard core cabin crew extremists.
An agreement now that allows BASSA to stay and fight another day is not in BAs interest.