If you are operating as a licensed cinema, you may only screen certified films and must display the certificate in advertising, at the entrance and on screen immediately before the film.

Films in this country are certified by the British Board of Film Certification and categorised as follows:
 

U, Uc – Universal
PG – Parental Guidance
12 – Children under 12 are restricted
12A – Children under 12 to be accompanied by an adult
15 – Children under 15 are restricted
18 – Children under 18 are restricted
R18 - Explicit adult material, restricted to specially licensed cinemas and retail premises

However, this certification has no legal status; the ultimate power rests with the local authority, which may choose to ignore the classification. Local licensing conditions differ from area to area, but most have adopted a common approach that requires films screened in licensed premises (e.g. cinemas) to be certified by the BBFC, and forbids venue managers from admitting younger persons into films with an age restriction.

This may cause problems for societies who wish to screen uncertified material (e.g. films by local filmmakers or even schools) in cinemas or other licensed premises but in general local authorities will not intervene unless the film is likely to “cause a public disturbance”, “breach of the peace” or “deprave or corrupt a significant majority”!

If in doubt with unlicensed material, contact your local Licensing department and see if they will grant a specific local certificate for the film. Note that full membership of a closed film society (ie, one that does not require a cinema licence) is not available to under-16s.