OP,
I can't imagine your upset and devastation that they have effectively bought your child.
She is still a child and it could take several years for her to recognise the manipulation and coercive behaviour that she is being subjected to.
There is absolutely nothing you can do.
She is a teen having money thrown at her.
It is that simple and it is appealing.
I have knowledge of something similar involving a marriage some years ago.
You need to box up your emotions, swallow your grief, and box clever.
Of course you can express surprise, BUT you understand that if this is what she wants to give her the best A levels etc, then you wish her the very best.
They have money and they want to hurt you.
You can't win.
What you can do is not add to their alienation of her from you.
Don't let them see your devastation.
If she is more like them than you, then she may well be bought permanently.
But if she is a good person, then hopefully she will mature and restart a relationship with you.
I am so sorry for you.