Is that the one where some guy had his balls nailed to a floor (with his consent) by other guys but they were all convicted of assault anyway?
There is a significant difference in that it was a criminal prosecution for assault, even though the "victims" were seemingly willing and had made no complaint. A criminal offence is effectively seen as a crime against the state, rather than against the victim and hence is prosecuted as Regina vs whoever.
Nevertheless as someone has already alluded to, in the UK, you simply can't sign away your rights to damages under tort law, so waivers are less applicable than they possible are in the US. A waiver may still have some relevence that you've acknowledged that you are engaging in a risky activity but there is likely still a "duty of care" for some aspects even if there's not a blanked guarantee of safety
Tort case law can seem contradictory. Here are some I remember from Clerk and Lindsell. The gist is right but I may be sketchy on details
A householder sued a cricket club for broken windows but they lost, as it was held that cricket was vaguely a good thing and so broken windows were basically just bad luck
Another case <whoever> vs Cheltenham Cycle Club (a motorcycle club here) . They'd organised a motorbike race and the injured (maybe killed?) party was a spectator hit by a racer going off the course. It was held under "violenti" that racers coming off was a risk the spectator had willingly accepted.
Conversely in another case a landowner had tolerated ("suffered" was the word used) a bunch of kids playing football on a bit of waste land and a stray ball knocked off a moped rider who was killed. It was ruled the landowner was liable (under occupier's liability law) as he'd not prevented them playing there. To me this seemed wrong in law given what the rest of the book said , and one of the criteria used is "policy" whereby they might have ruled that kids playing casual football was generally a good thing. But anyhow, that was how the case went, however unfair it may seem for the landowner