Yep, phone childline, or if you can't use the phone, email Samaritans. It really does help and they are more experts than us. Physical abuse is illegal and you should really so something about it. You could also call the police, and if you think talking to her won't work, just don't make contact with her much. Tell a friend and they can help you. I think you should ask Childline or Samaritans, it will help.
http://www.childline.org.uk/
And the Child abuse page is
http://www.childline.org.uk/Childabuse.asp
http://www.samaritans.org.uk/
I really do hope this helps. No kid deserves to be hit by their parents, and especially for no reason at all.