Again, they won't agree to this either.
They might. I don't think this has been put in front of them. It's possible that they are idiots, and when explained to them how shitty their ask looks from Hovis's perspective, they might be open to some kind of negotiation.
But if not they are just dicks.
It's really ducking simple though, you explain that your offer was based on the information you had at the time and the assumption you'd be in before Christmas. Which is reasonable.
And take it from there. It might be that you can still be in before Christmas because they want to sell and most of the evidence presented suggests they do.