The BBFC (British Board of Film Classification)

The BBFC is an independant, self-financing and not-for-profit media content regulator. operating transparent, consistent and trusted co-regulatory and self-regulatory classification and labelling systems in the UK.
Their mission:

• protect the public, especially children, form content which might raise harm risks.
• empower the public, especially parents, to make informed viewing choices.
• recognise and respect adult freedom of choice within the law.
• respond to and reflect changing social attitudes towards media content through proactive public consultation and research.
• provide a cost-effective, efficient classification service within our statutory remit.
• work in partnership with the industry to develop innovative service models to provide content advice which support emerging media delivery systems.
• provide an effective service to enforcement agencies.

• No one younger than 15 may see a ‘15’ film in a cinema. No one younger than 15 may rent or buy a ‘15’ rated video work.
• The work as a whole must not endorse discriminatory language or behaviour.
• Drug taking may be shown but the film as a whole must not promote or encourage drug misuse. The misuse of easily accessible and highly dangerous substances (for example, aerosols or solvents) is unlikely to be acceptable.
• Strong threat and menace are permitted unless sadistic or sexualised.
• Dangerous behaviour (for example, hanging, suicide and self-harming) should not dwell on detail which could be copied. Easily accessible weapons should not be glamorised.
• There may be frequent use of strong language (for example, ‘fuck’). The strongest  terms (for example, ‘cunt’) may be acceptable if justified by the context. Aggressive or repeated use of the strongest language is unlikely to be acceptable.
• Nudity may be allowed in a sexual context but without strong detail. There are no constraints on nudity in a non-sexual or educational context.
• Sexual activity may be portrayed without strong detail. There may be strong verbal references to sexual behaviour, but the strongest references are unlikely to be acceptable unless justified by context. Works whose primary purpose is sexual arousal or stimulation are unlikely to be acceptable.
• No theme is prohibited, provided the treatment is appropriate for 15 year olds.
• Violence may be strong but should not dwell on the infliction of pain or injury. The strongest gory images are unlikely to be acceptable. Strong sadistic or sexualised violence is also unlikely to be acceptable. There may be detailed verbal references to sexual violence but any portrayal of sexual violence must be discreet and have a strong contextual justification.

• No-one younger than 18 may see an ‘18’ film in a cinema or buy/rent an `18’ rated video/game/DVD.

In line with the The Human Rights Act 1998, at ‘18’ the BBFC’s guideline concerns will not normally override the principle that adults should be free to choose their own entertainment. Exceptions are most likely in the following areas:

• where the material is in breach of the criminal law, or has been created through the commission of a criminal offence.
• where material or treatment appears to the BBFC to risk harm to individuals or, through their behaviour, to society – for example, any detailed portrayal of violent or dangerous acts, or of illegal drug use, which may cause harm to public health or morals. This may include portrayals of sexual or sexualised violence which might, for example, eroticise or endorse sexual assault
• where there are more explicit images of sexual activity which cannot be justified by context. Such images may be appropriate in ‘R18’ works, and in ‘sex works’ (see below) would normally be confined to that category.

Where sex material genuinely seeks to inform and educate in matters such as human sexuality, safer sex and health, explicit images of sexual activity may be permitted.
Sex works are works whose primary purpose is sexual arousal or stimulation. Sex works containing only material which may be simulated are generally passed ‘18’. Sex works containing clear images of real sex, strong fetish material, sexually explicit animated images, or other very strong sexual images will be confined to the ‘R18’ category. Material which is unacceptable in a sex work at ‘R18’ is also unacceptable in a sex work at ‘18’.

http://www.bbfc.co.uk/ (I have used information directly from this website)