Bill Thompson writes about the London obscenity squad in the 1990's. For the longest time, they were failing to get Parliament to ban _soft porn_ (never mind spanking) but couldn't get any support. Finally they busted a few gay guys in Spanner, an operation that was originally justified as an investigation of a murdering Satanic cult but found no such thing. They used a videotape of long time gay associates paddling and wax-dripping, charging them successfully with assault. Apparently, after that they kept their budget healthy and felt justified in raiding parties and confiscating leather pants as items likely to inspire violent crime.
Historically at least, there are common points with American situations. Both countries have come up with higher profile cases involving gay men, and with suggestions that an SM interest is likely to lead to extreme bodily harm for participants -- if not child snuff. Both an American case in the 1960's and the Spanner case resulted in rulings that essentially, mentally sane adults cannot legally "consent" to the bottom role in SM activities. (Not sure the 1960's case is in Thompson, it may be from other work I've read.) The defense attempted in both cases to point out that popular sports involve much more physical danger, and both courts rejected that with assertions that SM does not have certified referees.
Thompson also discusses the linkage between certain feminists, Christian and psychoanalytic outfits that lined up against SM in both the US and UK, with considerable communication and influence across the ocean.
For gory and wacky details:
Bill Thompson 1994 Sadomasochism: Painful Perversion or Pleasurable Play? London: Cassell
Also, a quick reference to Spanner (based on Thompson), some similar issues and research on the London scene by Andrea Beckmann... 2001 “Deconstructing myths: the social construction of ‘Sadomasochism’ versus ‘subjugated knowledges’ of practitioners of consensual ‘SM’ ” Journal of Criminal Justice and Popular Culture 8(2) pp.66-95. Albany, NY: Univ. at Albany
That last one is online
(I think you can switch the HTML for PDF in this address if you like, but my Acrobat is being odd just now):http://www.albany.edu/scj/jcjpc/vol8is2/beckmann.html