From Noggin The Nog:

The post about the BBFC classification is fascinating. The following is the guideline from the BBFC web site which indicates what is permissible in a PG rating. It is interesting to note the specific references to “fantasy” as being a mitigating or acceptable reason for violence and horror. The type of violence which must surely form part of FOTR if shown in the context of a contemporary backdrop would almost certainly have earned a 12 classification.. Recently, the most likely scenario in the UK has been for cuts to made in films rated as PG-13 in the US in order to obtain a 12 classification in the UK. Alternatively the distributors have left the film uncut and accepted a 15 classification. Recent examples of this include Tomb Raider, Charlies Angels and Mission Impossible II. As FOTR is expected to have a PG-13 rating in the US I was ,if anything, expecting cuts. It can only be the fantasy context which has obtained this suprise decision.

‘PG’ General viewing, but some scenes may be unsuitable for some 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.

Horror – Frightening sequences should not be prolonged or intense. Fantasy settings may be a mitigating factor.
Imitable Techniques – No glamorisation of realistic, contemporary weapons. No detail of fighting or other dangerous techniques.
Theme – More serious issues may be featured, eg crime, domestic violence, racism (providing nothing in their treatment condones them).
Violence – Moderate violence, without detail, may be allowed – if justified by its setting (eg historic, comedy or fantasy).