If industrial action is taken outside the protected period, the employer can take action against both the union and the strikers. BA could, for example, sue Unite for the costs of the strikes and discipline the strikers.
I can certainly see BA suing Unite, but it is much less clear that strikers would be sacked as this would maintain the running sore between Unite and BA. However, I won't be at all surprised if some form of lesser action is taken, such as removing ST again.