The legal action is a simple tactic to delay industrial action beyond the 12 week point at which legal dismissal of those involved is allowed with no come back.
Though that would require BA to lose the injunction.
If Unite have any sense they will call off next week's industrial action pending the result of the injunction. They are exposed already and I doubt they'd want to bet the farm on being on the hook for more damages.
If BA gets an injunction, Unite will conduct a fresh ballot then this farce will carry on for another few months.
If BA does not get an injunction then, as you say, the clock starts ticking towards the 12 week deadline.
Whatever the motives behind this, I doubt (as the BASSA acolytes would like to think) it is because BA is panicking.