It's already the law in England & Wales that you can't consent to injury above actual bodily harm. That was decided by the House of Lords in R v Brown. Putting it into statute makes absolutely no difference to the strength of the law as currently stands. The way that rough sex could be relevant is that to commit murder someone you need to intend to do them grievous bodily harm or to kill them. If you don't have that intention then what you did could still be manslaughter i.e. a crime, but it won't be murder. So someone could argue that they didn't intend to hurt the victim because they were just having sex. It's up to the jury to decide whether they believe them or not, relying on evidence (which might pretty clearly point to the defendant intending to hurt the victim, as in Grace Millane's case). So, if there's a problem here it's probably in societal attitudes, rather than the law.
Interestingly enough, the Owen Bowcott article seems to imply that the victims in the R v Brown case were women, when in fact they were men. It made for pretty horrific reading when I was at law school, and is often regarded as a homophobic decision. Interesting to see it spoken about as a good thing.