how come this is prosecuted as ABH when the victims consented, yet when women are harmed their "consent" means the man who harmed them gets off?
I believe (though IANAL) that under E+W law, you cannot legally consent to be the "victim" of physical abuse, where that physical abuse crosses the legality line. So if you're in an S+M relationship, you can legally consent to be on the receiving end of violence that doesn't qualify as ABH/GBH, but no more than that. Happy to be corrected if I'm wrong.
(I'm sure there are legal guidelines as to when something crosses these lines - I'm also sure that there must be a lot of blurring, and even mose sure that I'm glad I have no input, professional or otherwise into this sort of stuff!)
The second part of the OPs question is the more interesting part --