Scooby, I fear the wording of the legislation is such that Virgin Atlantic could arguably claim that they have been negotiating since April and have failed to reach a resolution with those employees who found it necessary to strike.
Strikers are technically in breach of contract, so as long as they follow the 12 week rule, there seems little to stop them showing the door to them just as soon as they can (and if you can feel the temperature of the feelings in The Office, I wouldn't be at all surprised).