http://www.bbfc.co.uk/general/
From this site and the guidelines they set out I have decided that as our film contains Very strong language, violence and possible drugs and sex references it may have been possible to be rated as a 15. However I feel that as the theme of the film is about crime this could mean there is a chance that it will glamorise crime and gambling. As well using themes that could be immitatable.
So after looking at the guidelines set our by the BBFC I have decided that our film will have a certificate of:

Due to Very strong Language, Violence, drug and sex references and a criminal theme.
This means our target audience is Adults over 18. Most likely young adults.
It has to be taken into consideration that even though the opening scene we will create may not depict eveything I have mentioned, the rest of the film will contain the content so the film should only be viewed by an adult audience.