Do they have Alateen in the UK? You may want to look into taking her to meetings.
There are also reading materials that are written for her age group to help her understand the disease of alcoholism. Here is one link and I know there are more materials out there.
Coping With an Alcoholic Parent
Just as adults, teens need to learn about the disease to help them to cope and see that the problems and feelings she is having about her mom and how she acts have NOTHING to do with her. She needs to know that her mom loves her but that she is sick and unable to care for her the way a healthy mom would. I would not leave her with the mother unsupervised for any amount of time. No matter how mature the child seems to be, or how well she appears to cope with the issue of her mother, or with her mother directly, she still needs help dealing with this. We all need help dealing with this.