In law, you cannot consent to something that will cause serious harm. Which is designed, in the most part I believe, to prevent the defense of "it was just rough sex". There was a case I think I remember where two lads filmed themselves putting spanners and all sorts up their urethras and they were both convicted.
You might be thinking of Operation Spanner in 1987, which provided case law for this sort of thing. There was one feller who nailed someone's knob to a plank. Sandpaper on the bellend was also popular.
In law, you cannot consent to something that will cause serious harm. Which is designed, in the most part I believe, to prevent the defense of "it was just rough sex". There was a case I think I remember where two lads filmed themselves putting spanners and all sorts up their urethras and they were both convicted.