You can be sacked any time for absolutely no reason at all. It might be illegal, but you can still be sacked. Just like it might be illegal to go on strike without having followed the proper process, but that has not stopped various groups within BA in the past.
If you are sacked illegally, you can claim compensation and reinstatement through an employment tribunal. If the company refuse to reinstate you, they have to pay a fine. In total, the maximum payoff an individual crew member would get, would be a few tens of thousands; let's say 30k (although I suspect it would be lower in reality). 2000 x 30k = 60Million - About as costly as a couple of days striking, but I bet the strike/sick out would be broken pretty quickly once word spread of 2000 people being sacked. The fact that the company actually wants to lose 2000 crew has put a whole new spin on things. The crew can no longer hold the company to ransom, believing that they cannot train replacements quickly enough, because they do not need to!
At the end of the day, the company cannot afford to be held to ransom by any workgroup or it will quite simply not survive. The unions need to negotiate according to the current need, or in my opinion BA's hand will be forced.