The UK has now passed a law to criminalize the possession of "violent" and "extreme" pornography. A mother who blamed her daughter's murder on pornography involving strangulation campaigned for the law. You can read the language of the law here, you may also find of interest this website, of a group opposing the law. The most pertinent language is this: covered material includes material that "explicitly and realistically" depicts "an which which threatens a person's life," or "an act which results, or is likely to result, in serious injury to a person's anus, breasts or genitals."
It strikes me that the terms "threatens a person's life," "likely to result," and "serious injury" make it possible to interpret this law as criminalizing large chunks of BDSM pornography, i.e., material that is produced by and for consenting adults and that (though it may technically fit the above definition) when understood in context, is not intended to be viewed as depicting and does not otherwise promote actual violence. Examples: erotic knifeplay, simulated choking, and the ever-popular CBT (not cognitive-behavioral therapy - look it up).
Particularly remarkable is that, unlike U.S. obscenity laws, but like child pornography laws, the law criminalizes not only production and distribution but possession. Having forbidden images on your hard drive could land you in jail.
This raises in my mind the applicability of the UK Human Rights Act and through it the European Convention on Human Rights. These documents have not been consistently and broadly applied in the area of sexual privacy - witness the European Court's upholding of the Spanner convictions.
Interestingly and disturbingly, the UK Government cited the Spanner case in its justification of the law:
The material to be covered by this new offence is at the most extreme end of the spectrum of pornographic material which is likely to be thought abhorrent by most people. It is not possible at law to give consent to the type of activity covered by the offence, so it is therefore likely that a criminal offence is being committed where the activity which appears to be taking place is actually taking place. The House of Lords upheld convictions for offences of causing actual and grievous bodily harm in the case of Brown  1 AC 212 which involved a group of sado-masochists who had engaged in consensual torture. The threshold that the clauses have set is very high, so while those taking part might argue that they had consented to it, such consent is not valid at law.
Once one concludes - as the House of Lords did a decade ago, that BDSM is itself a crime, criminalizing its depiction, and even possession of such depictions, becomes easy. The justification, vis a vis the right to freedom of speech, is identical to that regarding child pornography: the forbidden material is the record of a violent crime, and its dissemination furthers the harm to the victim of that crime, and feeds a market for further crimes. This is the rationale on which the US Supreme Court has upheld the child pornography laws, and without knowing the corresponding case law in the UK or the European Court of Human Rights, I would expect a similar result here when the law is, inevitably, challenged in court.
Note, however, that the threshold for application of the new UK law is a) that a "reasonable person would think" the material depicted an actual person, and b) the depiction of violence is "realistic." Thus, there need not be actual violence to an actual person. Here the similarity to US child pornography laws ends. Our Supreme Court has clearly said that "virtual child pornography" is protected by the First Amendment - criminal laws can only cover the depiction of actual sexual activity with actual minors. This ruling has been criticized on the ground that, given the "beyond a reasonable doubt" standard and the fast-evolving state of imaging technology, it simply makes prosecutions too hard to procure. And this was undoubtedly in the minds of the Parliament here. In any event, this provides an significant distinction between both the child pornography and Spanner cases and this new law.
It will be very interesting to see what becomes of this law. One thing, however, is clear: it will not prevent a single violent crime.