Here’s exactly what the BBFC said about the film and their decision:
“The principal focus of ‘The Human Centipede II (Full Sequence)’ is the sexual arousal of the central character at both the idea and the spectacle of the total degradation, humiliation, mutilation, torture, and murder of his naked victims,” the BBFC wrote.
“Examples of this include a scene early in the film in which he masturbates whilst he watches a DVD of the original ‘Human Centipede’ film, with sandpaper wrapped around his penis, and a sequence later in the film in which he becomes aroused at the sight of the members of the ‘centipede’ being forced to defecate into one another’s mouths, culminating in sight of the man wrapping barbed wire around his penis and raping the woman at the rear of the ‘centipede.’
“There is little attempt to portray any of the victims in the film as anything other than objects to be brutalised, degraded and mutilated for the amusement and arousal of the central character, as well as for the pleasure of the audience. There is a strong focus throughout on the link between sexual arousal and sexual violence and a clear association between pain, perversity and sexual pleasure.”