Not sure how they could use "discrimination" of any kind as it was stated well before the strikes started. As has been said, it's not part of the contract, it's a "perk" that can be removed at any time at the whim of the company.
If they, Bassa/Unite, did try to go to court over this I reckon they would get short shrift even if they went down the "constructive dismissal" route at an Industrial Tribunal. Let's face it, BA's lawyers will have made sure that things were water and airtight before allowing the decision to be made.
It's another hiding to nothing, especially with Bassa's track record in the court