According to the BBFC:
http://www.bbfc.co.uk/ - British Board Of Film Classification
A 15 certificate rated film always has a theme, but must be suitable to the audience. Hence fourth no theme is prohibited, provided the treatment is appropriate to 15 year olds.
Language is also infrequent, with a few references to strong language, and the strongest language is only justified if it is in context. Continued aggressive use of the strongest language is unlikely to be acceptable.
Nudity is allowed in sexual context, but no strong detail is shown. There are no constraints on nudity in a non-sexual or educational context in a 15-certificate film.
Linking with nudity, sexual content is also shown and is allowed to be portrayed, but with no strong detail. However there may be strong verbal references to sexual behaviour.
Violence can be strong but with no focus or dwell on the infliction of pain or injury that has been caused. Scenes of sexual violence must be discreet and brief.
Dangerous techniques e.g. combat, hanging, suicide, weapons that are easily accessible and self-harming should not be focused on to much in imitable detail. Easily accessible weapons should also not be glamorized or portrayed as ‘cool’ in order to not influence the audience.
Very little gore is shown within horror, but strong threat and menace are permitted. The strongest gory images are unlikely to be acceptable.
Drugs and drug taking may be shown but the film as a whole must not promote or encourage drug misuse in anyway and once again much no glamorize this or portray it in a way that looks acceptable in order not to influence the audience.
BBFC findings show that at the age of 18 a person should be entitled to see whatever they wish as they are an adult. However exceptions include the promotion of dangerous acts (both to the individual and society). certificate 18s also cannot risk harm to individuals or to society, or involve sexual violence that could support sexual assault. Explicit sexual content is only shown if the context can justify it.
The difference between a certificate 15 and certificate 18 film.
The main contrasts between an 18 certificate film and a 15 certificate is the detail and the explicitness of the context. In a an 18, sexual content is much more explicit and violence is in much more detail. Strong gore and violence is more obvious, in a lot more detail and more extreme in an 18, along with the use of very strong language. Even so, in both a 15 and 18 the content must be justified by the context.