Safe words
That is quite a ridiculous case. one could argue that ear piercing, or "worse" is consentual harm to teh self. One could argue that doing sports such as a marathon, or an iron man, where your body is pretty much torn up, is consectual harm that should therefore be outlawed. And i suppose no one is allowed to fight in the UK in the ring?
Right. being in a ring with some other guy making your face into jello is "macho" and "cool" and "perfectly normal". but asking someone to spank you is not.... gotcha.
Piercing has always been a very odd one, because it's effectively legal to piece anywhere for jewelery, but to do it as a temporary thing for a corset piercing or just for kicks is... different? As one person said in a blog somewhere "you can break your skin because it looks nice, but not because it feels nice".