(Even without the way that it's one place, there's a case that says a brothel need not involve sexual intercourse, so given that no-one has seriously tried to argue that BDSM services aren't 'sexual' under the Sexual Offences Act 2003, there's no legal difference between renting out the bedroom and renting out the dungeon. There are places where it might make a difference in terms of the chances of anyone caring, but you'd have to be very unlucky to get into trouble in London, for example. As ever, don't annoy the neighbours...)