The fact that they can sack you, and the fact that the law cannot make them reinstate you even if it was found to be illegal is kosher. The amount of money they would have to pay is also kosher, and is based on info that BALPA sent out during the Openskies dispute.
BA's intention to impose at the end of the month if no satisfactory negotiated agreement is reached is also kosher.
The idea that BA will sack the first 2000 who do not show up for work - that is just my opinion, and it is based on what I would do if I was leading BA. To be honest, I am not sure how many other options Willie actually has.