As cinema grew into a mass medium it was inevitable that moral guardians would take an interest. In the early days, censorship was a local issue with councils deciding what was was fit to be seen. This made life difficult for film makers and distributors who had to take account of various petty rules and foibles. Far better to tone films down to a level that was broadly acceptable to the mass of people and to certify they were suitable.
Thus was born the British Board of Film Censors, an industry body which classified films (and cut out offending material). These classifications had little legal status and could be overruled by local authorities, but they were largely respected. In 1984 the organisation changed its name to the British Board of Film Classification.
U - Universal (unrestricted admission)
A - Adult: (Children accompanied by responsible adult)
U - Universal (unrestricted admission)
A - Adult: (Children accompanied by responsible adult)
H - Horrific (Persons over 16 only)
U - Universal (unrestricted admission)
A - Adult: (Children accompanied by responsible adult)
H certificate films were banned (presumably it was felt that life had enough horrors)
U - Universal (unrestricted admission)
A - Adult: (Children accompanied by responsible adult)
X - Persons under 16 not admitted
U - Universal (unrestricted admission)
A - Parents advised that the film may be unsuitable for children under 14
AA - Children under 14 not admitted
X - Persons under 18 not admitted
U - Universal (unrestricted admission)
PG - Parental Guidance. some scenes may be unsuitable for young children
15 - Passed only for persons of 15 years or over
18 - Passed only for persons of 18 years or over
U - Universal (unrestricted admission)
PG - Parental Guidance. some scenes may be unsuitable for young children
12 - Passed only for persons of 12 years or over
15 - Passed only for persons of 15 years or over
18 - Passed only for persons of 18 years or over
This is the point that things get really murky. If a Hollywood production has enough money behind it, and a target audience with a lower age than the certification classification would allow, then they just create a new one specially. (i.e. Lord of the Rings: Fellowship of the Ring's catchy rating "PG Fantasy violence and battle scenes may not be suitable for under 8's")
The 12 rating is revised to a 12A which allows the under-twelves to get in with an adult. But there will be a warning under the certificate explaining what type of naughtiness might be in the film.
