BBFC

The British Board of Film Classification (BBFC), is a non-governmental organisation, funded by the film industry and responsible for the national classification of films within the United Kingdom. It is an independent, non-governmental body which has classified cinema films since it was set up in 1912 and videos/ DVDs since the Video Recordings Act was passed in 1984.
The BBFC rates theatrically released films, and rated videos and video games.

U - should be suitable for audiences aged four years and over. ‘U’ films should be set within a positive moral framework and should offer reassuring counterbalances to any violence, threat or horror.

PG - General viewing, but some scenes may be unsuitable for young children. Unaccompanied children of any age may watch. A ‘PG’ film should not disturb a child aged around eight or older. However, parents are advised to consider whether the content may upset younger or more sensitive children.

12/12A -  the material is suitable, in general, only for those aged 12 and over. Works classified at these categories may upset children under 12 or contain material which many parents will find unsuitable for them.
No-one under the age of twelve may be permitted to see a twelve rated film at the cinema, unless the parents or guardian thinks the film is suitable for their child and accompanies them.

15 - No-one younger than 15 may see a ‘15’ film in a cinema. No-one younger than 15 may rent or buy a ‘15’ rated video work.

18 - No-one younger than 18 may see an ‘18’ film in a cinema. No-one younger than 18 may rent or buy an ‘18’ rated video work.

R18 - The ‘R18’ category is a special and legally restricted classification primarily for explicit works of consenting sex or strong fetish material involving adults. Films may only be shown to adults in specially licensed cinemas, and video works may be supplied to adults only in licensed sex shops. ‘R18’ videos may not be supplied by mail order.

This information was taken from the official BBFC website