Can my dependent 17 year old move out without parental permission?

I think keeping open communication with her is vital, whether she's decided to stay at home or not. She's well within her rights to move out, and battling to change her mind if she tried again may just cause havoc for your relationship. Make your opinions clear, but trying to force her to stay would just make her want to go more. If her boyfriend really is as bad as you suspect then the chances are their relationship won't last.

Spend some quality time together and let her know that you're there for her no matter what.

I moved out at 16, and moved quite a distance away. My parents made it clear that they didn't want me to, but supported me because they knew it was what I wanted and I had a good relationship with them. I found my feet quite easily and don't regret it at all, infact I'm much better off than a lot of my friends who are still living at home now.

I'm not sure about child benefit now, but when I moved out my parents notified them straight away and they still paid it up until the next 'segment' of payments, which was the earliest it could be stopped. I believe - but I could be wrong!