The BBFC (British Board of Film Classification.) is designed for age ratings for TV/Film.
This blog will explain each age ratings and what is entitled before it breaches to the upper age certificate.
U/universal ratings: This age certificate is designed for everybody, for children, teen and adults. Being universal, there is zero tolerance to high violence, drugs and mature language, etc. However depending on how the violence is portrayed on screen, the piece must be very tamed so that it will not offend children and parents.
PG/Parental guidance: This age certificate is for general viewing, however some scenes may offend children, so parental guidance is provided for a child to see this movie.
12A rating: When the age certificate reaches to 12, moderate violence, sex references and language is acceptable depending on the themes of violence and how the language is used. When spider-man was released in 2002, many children were craving to watch it in the cinema, however because it was rated 12, many of the public were disappointed. This made BBFC decide to invent a new age rating called 12a, acceptable for people under 12 only if they have parental guidance. References of drugs and suicide is acceptable but not in detail, where young people will be able to copy from it.
15 age ratings: 15 rated films/TV can show scenes of strong sex references, strong verbal references to sex, strong violence, strong language, hard use of drugs. Strong violence is acceptable, but strong gore or sadistic violence will draw the line.
18 age rating: An 18 rated TV/Film will include pretty much anything, from hard drug use detailed strong sex scenes (Real), to sadist violence, strong gore, strong language such as F**K and extreme strong language such as C**T is acceptable. However if the whole piece has been shown to encourage or misuse of drugs, it is zero tolerance.
R 18: You can only watch restricted 18 if the cinema is licensed for it, also may find them in licensed shops. This is because the material is to disturbing or offensive to the general public.
This is the link to show my research http://www.bbfc.co.uk/what-classification/18