What would happen if a surrogate Mother wanted to keep the baby?

Are there any laws (in the UK) that would prevent her from keeping it if she'd promised it to someone else, or if it was someone else's egg, or does she have rights to it because she gave birth?What would happen if a surrogate Mother wanted to keep the baby?
Her body, her baby.What would happen if a surrogate Mother wanted to keep the baby?
This has happened a few times and the surrogate usually gets to keep the baby