The British Board of Film Classification (BBFC) is responsible for classifying films in the UK according to their content. They use a rating system with 7 symbols ranging from universal to restricted. To determine a rating, the BBFC examines factors like discrimination, drugs, language, nudity, sex, violence and more. For the film Fish Tank, the BBFC gave it a 15 rating despite it bordering a 18 due to strong language, sex references, drug use and violence. They considered the context and impact of these elements on younger viewers to determine the appropriate rating.