You can't physically stop him if there's no legal document stipulating rules and preferential rights that you have over him I think.
The consequences in terms of a school fine should be paid by him. That part is essential. That he pays not you. And school know that if this goes ahead.
I have to work with this behaviour in my life from multiple angles. I understand the experience of it.
Yes it's a performance.
But, does your daughter want to go? Could she actually enjoy this experience?
Will her life be ruined going away a week before mocks? I know it's not ideal but they are mocks, not the real thing.
How much is this you without realising just showing strength against a very difficult manipulative character and not really about how vital it is she's in school that week? ( This is normal btw. I live this stuff. )
Do you need any from this guy? Does he give you money or help in other ways?
Agreeing to this pathetic performance with ' you know what, that holiday sounds great for her. There might be a school fine but hopefully they won't. I will have to email them to confirm it's your decision not mine. But I think a holiday is a good idea now I've thought about it.
Can you just let me know exactly when she's going and exact plans for coming back?'
This might be a good thing to do for you and however he responds to a bit of fluffing.
How does the above feel?
Is this fight really worth it? She might have a great time. And you get a week for yourself. Win , win to me OP. And you also are not responding to the baiting. You win then