Details of the audience - certificate for our film

I looked on the BBFC website (the British Board of Film Classification). 

For the type of film we are wanting to make, we were thinking the film would be a 15. 

15 british board of film classification research

We think our film would be a 15. We think this because a lot of psychological thrillers that we have looked into are usually 15's. Also because we are wanting to use horror references and violence with wounds and blood.
If our film was a U or a PG, a lot of people would think it would be a child's film and it wouldn't fit in with our ideal story line.
If our film was a 12/12a, we would be allowed to use mild language and violence but it would be quite difficult because of the genre we are wanting to do.
If the film was an 18 then even fewer people would come. Firstly because of the audience age range and also whether or not they'd be interested in what the plot was about. But we would be able to add more effect to detail on injuries and the aspect of drug use.