Here is a statement released by the BBFC, which goes on to not only divulge graphic details about the plot, but also gives their rationale in handing down the ban. Be warned though, there are plot spoilers below, and graphic ones at that, so if you're squeamish or don't want to know the plotline of this sequel, you can skip to director Tom Six's reaction to the ban below the BBFC statement.
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. 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. It is the Board's conclusion that the explicit presentation of the central character's obsessive sexually violent fantasies is in breach of its Classification Guidelines and poses a real, as opposed to a fanciful, risk that harm is likely to be caused to potential viewers.
David Cooke, Director of the BBFC said: "It is the Board's carefully considered view that to issue a certificate to this work, even if confined to adults, would be inconsistent with the Board's Guidelines, would risk potential harm within the terms of the VRA, and would be unacceptable to the public."
Director Tom Six released a statement regarding the ban, which you can read below.
In related news, there has been no word on what cast members will appear in The Human Centipede II (Full Sequence) until now. Ashlynn Yennie will reprise her role of Jenny from The Human Centipede (First Sequence). Laurence Harvey will portray the new doctor who assembles this larger, 12-person centipede.
Dominic Borelli, Vivien Bridson, Lee Harris, Peter Charlton, Bill Hutchens, Dan Burman, Daniel Jude Gennis, Kandace Caine, Maddi Black, Lucas Hansen, Georgina Goodrick and Emma Lock all round out the cast, presumably as the new 12-person centipede.
The Human Centipede II (Full Sequence) was released October 7th, 2011 and stars Laurence R. Harvey, Ashlynn Yennie, Maddi Black, Kandace Caine, Dominic Borrelli, Lucas Hansen, Lee Nicholas Harris, Dan Burman. The film is directed by Tom Six.