The BBFC

http://www.bbfc.co.uk/about/mission-statement/

The BBFC produces detailed information about the content of every film and video game, which they call Extended Classification Information, or ECI. The ECI is designed to explain in greater detail the issues, and in many cases the plot lines, which led to the film being given the classification it is. As a result it may also contain plot spoilers, which is why you have to click on the link to open it because not everyone wants to see the spoilers if they haven't seen the film. All classification decisions are based on the BBFC’s published and regularly updated Guidelines. The Guidelines are the product of extensive public consultation, research and the accumulated experience of the BBFC over many years. They reflect current views on film, DVD and video game regulation.

http://www.bbfc.co.uk/classification/guidelines/the-categories/ 

This video show's the cuts the British Film Classification have made to Die Hard With a Vengeance to allow the film to fit within the certificate 15 guidelines. I like this video as it shows the original script and then what the BBFC has changed or cut it to in order to change it from a 18 certificate to a 15 to allow the company to reach their potential audience.

Video Recordings Act 1984

The most important piece of legislation to affect BBFC classification standards is the Video Recordings Act 1984. This Act requires all ‘video works’ (films, TV programmes or video games) that are supplied on a disc, tape or any other device that stores data electronically to be classified by the BBFC. Under this Act, the BBFC have ‘special regard’ to consider the likelihood of video works causing any harm to their potential audience, due to character's behaviour in society for example criminal behaviour, illegal drugs, violent behaviour or incidents, horrific behaviour or incidents and human sexual activity. The BBFC needs to consider possible effects not only on children but also on other vulnerable people in society.