Last year one of the many nasties due to be blocked was websites which encourage people to develop eating disorders, aka 'Pro-Anorexic' content. The content doesn't appear to be blocked under any of the various filters, suggesting that the ISPs may have realised the problems in this area - namely that an unrealistically proportioned, air-brushed model on a mainstream fashion publication can unfortunately be as self-image damaging as any 'legitimate' weight loss tips.
With the situation as it stands today, in this issue, at least, ISPs seem to have decided that mental health in the UK is an issue more complex than one which can be addressed by attempting to block websites.
BT's (fully optional in this case) filter has already come under heavy criticism for blocking 'gay and lesbian lifestyle' content, before back-peddling to sex education only.
BT's filter whilst claiming 'not [to] discriminate between heterosexual and LGBT content' clearly blocks sites such as gaytimes.co.uk (but not pinknews.co.uk) and does in fact effectively block a range of sex education sites such as the sexual health charity http://www.fpa.org.uk and http://www.contraceptioneducation.co.uk
As this is (for now, at least) a fully opt-in category, the potential for wider damage is fortunately limited.