The date you are on about is when the 12 week strike mandate expires -when the mandate expires there is no legal protection from striking - UNITE will then have to re-ballot to carry on with industrial action that is all that date means.
It is not a date when BA can just 'sack' the strikers
Correct. However, it is also the point at which BA can serve notice on existing contracts if it chooses to do so. That will effectively render any subsequent ballot irrelevant as the law allows BA to assume that anyone who fails to sign a new contract has resigned.
Don't say you haven't been warned.