Bunch of slack-jaws. Real harm could be done to the viewer... So the classifiers are allowed to watch it and aren't sewing each other's shitholes to their mouths
http://www.totalfilm.com/news/human-centipede-ii-banned-by-bbfc
The highly controversial "A Serbian Film" was also heavily censored for UK audiences.
The biggest joke is that if I (A UK citizen) wanted to see this film, I'll just download the uncut version and so will thousands of others.
This type of niche torture porn isn't a big hit in the cinemas or something you'd casually pick out at blockbusters. Curiosity drives people to see it and they'll do what it takes (torrenting, newsgroups etc) to get it.
It's not the 1970s where word of mouth led you on a 3 hour car journey to purchase an imported version of "Cannibal Holocaust" from a man in a dark alley. "Banning a film" means nothing in current times.
Pretty much anyone in the Western World can see it days after it's release in the comfort of their own home
Parents basement