There are 6 main certificates for films according to the British certificates since 2009, they allow and forbid certain people to watch films due to age restrictions.
The certificates are the following:
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This certificate's Name is Universal. This means that the film is suitable for all ages and that the younger generation does not need to be accompanied by an adult. However before 2009 there used to be a certificate which was Uc. This was suitable for all, but especially suitable for young children to watch on their own. But because the certificate U pretty much covered this they scrapped this from 2009 onward.
The name of this certificate is Parental Guidance. This is for general viewing, but some scenes may be unsuitable for young children. So this leaves the parents to decide whether they accompany their child or not.

The name of this certificate is 12 Accompanied/ Advisory. This means that it is suitable for those aged 12 and older (cinema only) so this is only used in the cinema not on films or TV programmes that you watch/buy;under 12s admitted, but only if accompanied by an adult

The name of this certificate is just 12. This is suitable for those aged 12 and older (non cinema only) So this is not used in the cinema only when buying films etc.

The name of this certificate is just 15. This is suitable for those aged 15 and older and the person does not need to be accompanied by an adult. Sometimes if this were to be used in a cinema or when buying/renting a film then they may get asked for id.
The name of this certificate is just 18.It is listed on films and TV programs. and is suitable for those who are 18 an older. This probably because the scenes that are within the film may be unsuitable/violent for those who are at a younger age.