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.
Exactly my point Litebulbs. When the affected individual has to seek damages through the court system, won't the court require the individual to provide proof that he/she was genuinely sick? Wouldn't it have been easier to just provide the same proof at the time when BA requested it? This way the individual concerned will avoid further unnecessary lost time and expenses that a court case would entail?