The five appellants engaged in sadomasochistic sexual acts, consenting to the harm which they received; whilst their conviction also covered alike harm against others, they sought as a minimum to have their mutually consented acts to be viewed as lawful. None of the five men complained of any of the acts in which they were involved, which were uncovered by an unrelated police investigation.
The physical severity was not disputed. Each appellant (having had legal advice) pleaded guilty to the offence when the trial judge ruled that consent of the victim was no defence.
The question approved and certified as in the public interest on appeal was whether the prosecution had to prove (in all similar cases) a lack of consent on the recipient's part. The appellants argued against conviction under the Offences against the Person Act 1861 as they had in all instances consented to the acts they engaged in (volenti non fit injuria), that as with tattooing and customary-site body piercings their consent would be directly analogous to the lawful exceptions laid out by three cornerstone (and other) widely-spaced precedent cases.
The certified question of appeal which the House of Lords was asked to consider was:
Where A wounds or assaults B occasioning him actual bodily harm (ABH) in the course of a sado-masochistic encounter, does the prosecution have to prove lack of consent on the part of B before they can establish A's guilt under section 20 or section 47 of the Offences against the Person Act 1861?
The Lords by a bare majority, two out of five dissenting answered this in the negative, holding that consent could not be a defence to these (typically overlapping) offences.
Lord Templeman stated:
"It is not clear to me that the activities of the appellants were exercises of rights in respect of private and family life. But assuming that the appellants are claiming to exercise those rights I do not consider that Article 8 invalidates a law which forbids violence which is intentionally harmful to body and mind. 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. I would answer the certified question in the negative and dismiss the appeals of the appellants against conviction.
His judgment examined the acts to be "unpredictably dangerous and degrading to body and mind and were developed with increasing barbarity and taught to persons whose consents were dubious or worthless".
https://en.wikipedia.org/wiki/R_v_Brown
Basically arguing that BDSM shit is so cruel and unusual that it doesn't matter whether you consented to it or not.