As has been stated on here before - any employer, at any time, may sack any worker(s) for any reason, valid or not.
If its a non valid reason, as defined by UK employment law, the said workers may then have a claim for unfair dismissal. Such claim may well take months if not years to come to court, and the court, if upheld, will issue compensation. Such compensation will be comsensurat with the salary of the person and unlikeley to be more than 1 years salary. The court could instruct the employer to re-instate the employee but the employer can get around this by paying a fine and is not legally binding.
So, if it was identified there were 200 hardline militants, BA could remove them for good for around the cost of a days strike action (assuming £7m).
Of course, this would be a
PR disaster for BA, but it is a valid option.
There is no unlawful dismissal, there is no criminal or civil offence and no one goes to jail, its just a compensation issue, so a numbers game....