You're right, they cannot go behind your back and talk to them without your permission. The only time they can do this is if you have been sectioned/about to be sectioned. They have to inform your "nearest relative" if this is the case, and there's a set list for the order that works in. Even then you can apply to have the nearest relative changed although it isn't necessarily guaranteed, it depends if it is agreed it would be in your best interests to do so.

Even when I have been admitted to hospital in the past, the times I went voluntarily they couldn't inform anyone if I didn't want them to... it might be worth googling your rights and taking a printout to your CPN or something if she is still pushing for it. Sorry you are in this situation