BBFC or The British Board of Film Classification, the is an extract from their site
‘The BBFC is a trusted guide to media content. Working as an independent, self-financing and not-for-profit media content regulator, the BBFC operates transparent, consistent and trusted co-regulatory and self-regulatory classification and labelling systems’.
BBFC look for certain things in movies to judge them for the right age group, sexual content, weapons, explicit language and excessive violence.
PG or parental guidance is were the featured film can be viewed by anyone. best suited for those under 12 years old. However some scenes will require supervision for young children. this can vary from mild bad language or even no nudity.
For the consumption of those 12 and above. Use of drugs must be minimal, violence must not be excessive. and nudity must be brief and vague.
For those 15 and above. Miss use of easily accessible drugs may not be tolerated. Minor violence is tolerated. Scenes including self-harm must be extremely brief.