BBFC

The British Board of Film Classification is an independent, non-governmental body funded through the fees it charges to those who submit films and video works for classification. The BBFC classifies films on behalf of the local authorities who license cinemas under the Licensing Act 2003.

What determines a cirtificate 15 to an 18?

15 – Suitable only for 15 years and over No one younger than 15 may see or rent a ‘15’ film.
The cirtification is given when all aspects of the film are considered. they over look discrimination, drugs, horror, nudity, sex, violence, theme, language and Imitable behaviour to encourage restictions so younger veiwers are not disturbed or wrongly influenced by what they're watching.

Details of sex and nudity are kept to a minimum, yet there may be strong language throughout. weapons shouldn't be glamourised and little detail of dangerous behavior so it cannot be copied. The strongest gory images are unlikely to be accepted but violence may be shown frequently. showing the use of drugs is acceptable aslong as it the misuse of drugs is not encouraged.

Suitable only for adults No-one younger than 18 may see or rent an ‘18’ film. 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 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.

Other than this factor citificate '18' doesnt havnt have many restrictions of what the viewers are watching.
I found this information at http://www.bbfc.co.uk/