There are two elements to consent: whether the victim consented, and whether the defendant believes the victim has consented. The prosecution must prove that the victim did not consent and that the defendant did not believe in his consent: R v Donovan 
The general rule is that consent can only be available as a defence to assault and battery, AG’s Reference (No 6 of 1980), where two boys fighting saw one sustaining cuts and bruises. The other was guilty of s47 Offences against the person Act 1861, assault occasioning ABH.
Essentially fighting will always be an illegal act whether it takes place in the home or in private (Lord Lane CJ). But the rule was confirmed by the House of Lords in R v Brown, a case involving some really twisted individuals who caused each other injuries for sexual pleasure. They were charged with assault occasioning GBH under S47 of the Offences against the Person Act 1861, a frankly hopelessly outdated piece of legislation which mandates that ‘whosoever shall be convicted upon indictment for a common assault shall be liable, at the discretion of the court, to be imprisoned for any term not exceeding two years.
No doubt that when the case came to the public attention there was a great degree of interest in some of the more prurient sections of the media. As indeed there had been when the case was heard in the commons. The activity in R v Brown was not deemed to be a suitable exception to acts which would be consensual, such as sporting events. Lord Templeman argued that cases involving the defence of consent for sadomasochistic encounters came down to public policy.
if it is to be decided that such activities as the nailing by A of B’s foreskin or scrotum to a board or the insertion of hot wax into C’s urethra are injurious neither to A or B nor to the public interest then it is for parliament to declare them to be unlawful.
But the vast majority of the public are not sadomasochists, one suspects, and such judgements will always seem illiberal. They show the state’s interference in private affairs between consenting adults.
A similar judgement was reached in Emmet, where a husband tied a plastic bag over his fiance’s head in order to increase her sexual pleasure. Nothing illegal about that, although certainly risky. But then he poured lighter fluid on her breasts and set fire to it. The court saw this as violent conduct which moved beyond what could be consented to. It’s hard to see what pleasure could be taken to setting fire to someone’s breasts and in this case the courts came to the right conclusion.
But compare and contrast with R v Wilson, where a man (and it’s always a man) used a hot knife to brand his initials onto his wife’s buttocks, at her request. The court held that there was no difference between this type of branding, tatooing and similar personal adornments which are lawful activity, strange then, that they came to such a different conclusion in Brown.
These cases can be reconciled when you consider the type of acts that the law considers to be consensual and the ones that can’t be consented too. Emmet was successfully appealed because the relationship between the defendant and victim was a close one and the argument of consent was more compelling, whereas in Brown the parties were a club of homosexuals who met for the purposes of carrying out some sick activities, which the Lords were concerned would lead to the proselystization of the corruption of young men. Even if the acts were consensual, they were hardly those that any decent person would want to uphold.
Lastly, the decision in Slingsby led to the defendant being acquitted. The facts are similar to R v Boyea, in which the defendant inserted his fist into the victim’s vagina during sex. The defendant claimed that it was done for the purpose of sexual gratification. Held: violent conduct.
This was not the outcome of R v Slingsby, on the face of it the events were the same but for the fact that the defendant’s ring caused the injury and later death from septicaemia caused by the injury. There was no intention to cause harm, although it sounds painful in the extreme, but the courts acquitted him of unlawful act manslaughter on the grounds that he lacked the mens rea for assault or battery, i.e. the injury was caused accidentally during consensual sexual activity.
If you’re having sex and you decide to fist your partner’s vagina, you’ll probably be able to use consent as a defence, but if you’re a sadomasochistic homosexual engaging in mild torture you’ll find yourself on the wrong side of the law. What about drug addicts and alcoholics who abuse themselves, or athletes who put their bodies through years of agony? Marianne Giles calls the judgement in Brown “paternalism of an unelected, unrepresentive group who use but fail to recognise that power.”
It does appear to show heteronormative bias and one wonders whether the same judgement would have been reached if they had been heterosexual men.