The main question is how much have they been paid to go without a fight and to not talk to the press by slagging off Willy Walsh and the rest of the Board (although we know Kirkwood hates talking to the press).
Given how long Kirkwood has been at BA and subsidiaries (basically all his career) he would obviously go to an Employment Tribunal unless he was being offered a huge payoff or other guarantees. If they are being asked to take the blame and go without a fuss if I were them I would want at least three years salary and my pension contribution paid for the same period.
If they went to an Employment Tribunal I am sure they could prove that the malaise leading to the T5 problems extended much wider and went as far as the CEO and the Chairman themselves. They would point to huge corporate pressures from the two top men (CEO and Chairman) to say the Terminal would open on time no matter what and their, in effect, having no option but to go along with this and make the best of it.
They must have already been offered a cosy well paid transfer to somewhere else in the airline industry or massive payoffs not to fight this all the way. Especially when Walsh had previously said the buck stopped with him. Can you imagine the chairman of an Employment Tribunal questioning Walsh on that buck stops here point. The media would have an absolute field day.
By the way they obviously have not fallen on their swords. They are merely accepting an apparent public beheading in return for some kind of secret deal to protect their future if they agree not to go to a Tribunal hearing.