nononsense frank
BA can say and do what they want, at any time including stopping pay and making asumptions on who is and who is not on strike. It will be for the individual to then seek damages for unlawful deduction from wages, which is a breach of contract through the court system, which will no doubt lead to binding legal opinion.
It is not for BA to make the law, just defend its actions as a proportionate means to meet a legitimate aim. Obivously, if there is common law out there already, then it will be applied.