The BBFC

The British Board Of Film Classification

The British Board of Film Classification is an independent, non-governmental body, which has classified cinema films since it was set up in 1912, and videos since the passing of the Video Recordings Act in 1984. (www.bbfc.co.uk)

The BBFC sets the age limit of diffrent films such as 18 (Seen on Hostel) 15 (Seen on I am Legend) 12 (Seen on Bourne Identity) PG (Seen Monsters inc) U (Seen on Cinderella)

15:

15 rated certificate tend to allow moderate violence, gore and sexual activity. However not in explicit circumstances and it must be of relevance. Drug taking is not banned however it cannot be promoted to be seen in a positive way.

18:

18 rated certificates permit a wide range of gore, violence, sexual activity and drug taking (not to be promoted.) The difference between the two certificates lies not in context of themes; mainly in explicity.

http://www.bbfc.co.uk/downloads/pub/Guidelines/BBFC%20Classification%20Guidelines%202009.pdf

(The BBFC guideline PDF document)