CHAOS Tactics
My understanding is that all airport security passes are issued with a clause in the terms and conditions that clearly stipulate that the pass is only to be used for the purposes that they were issued.
Therefore should a cabin crew member use their security pass to gain access to an aircraft with no intention of flying on that aircraft (the reason for which the security pass was issued), then that would represent a significant security breach, and render both the individual and British Airways (who countersign all security applications) in breach of CAA rules.
Moreover, for the union to call people out on strike once they have entered a secure area would be wholly irresponsible, given that this would render the individual in breach of CAA rules and provide BA with a legal platform to re-assign any punitive CAA action against them on to the Union instead.