The British Board of Film Classifications states:
'All classification decisions are based on the BBFC’s published and regularly updated Guidelines. The Guidelines are the product of extensive public consultation, research and the accumulated experience of the BBFC over many years. They reflect current views on film, DVD and video game regulation.
In the most recent consultation over 8700 members of the public were asked for their views on classification – including consideration of issues such as language, discrimination, violence, sex and drugs in films, DVDs and video games, parental concerns about younger viewers and recent BBFC decisions. The Guidelines also take into account the various UK laws which the BBFC must consider and apply when making classification decisions'.

We are going to rate our film 15, this is because the British Board of Film Classifications states: 

Suitable only for 15 years and over

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.

Discrimination

The work as a whole must not endorse discriminatory language or behaviour.

Drugs

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.

Horror

Strong threat and menace are permitted unless sadistic or sexualised.

Imitable behaviour

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.

Language

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

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.

Sex

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.

Theme

No theme is prohibited, provided the treatment is appropriate for 15 year olds.

Violence

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.