There is a recent thread here on the "rough sex defence", and obviously you don't want people getting away with murder.
But to what extent, if any, should you be able to consent to harm in sex?
The UK standard appears to be that you can't consent to "actual bodily harm" which looks like theoretically a lot of BDSM could be illegal in the UK.
You probably wouldn't get charged in practice unless injury was getting more serious, but the actual standard (as far as I can tell) could theoretically mean that pretty mild BDSM without any risk to life would be illegal.
And the case law justification is along the lines that BDSM is sexually depraved and a perversion.
Compare with combat sports where you may potentially beat someone to death, and it's allowed because of consent.
https://en.wikipedia.org/wiki/R_v_Brown
"Society is entitled and bound to protect itself against a cult of violence. Pleasure derived from the infliction of pain is an evil thing. Cruelty is uncivilised."
The case law involved someone nailing their scrotum to a board, which is getting more serious and you might end up needing medical treatment for.