I'm not a lawyer, but there is the concept in English law of duress which can invalidate contracts that had undue pressure.

My point was a broader one not a legal one, that the company is acting in a manner that is hostile to a good work environment, putting up this kind of life-changing decision with only a few days to decide and then trying to paint decisions to stay as being a champion of the current direction.

That's treating your workforce with contempt.

Make the same offer with 30 days notice and see how many stay.