BBFC

Certification from British Board of film classification - Determines minimum age allowed to watch film - decision made by the content in the film. Connotes what will be in film and can appeal to target audience then.
In Iris, it will be a 15. I checked on the BBFC website and what's included in a 15 is what's in my film.
 15 - Frequent strong language, drug taking, Strong violence, portrayals of sexual activity, strong verbal references to sex, discriminatory language or behaviour. Iris includes scenes which are not suitable for children under 15 - drugs and in the film strong language. 

• Drugs can be seen however can not be promoted or encouraged.  There are scenes with minor drug usage and alcohol abuse.
• And the storyline contains sad scenes that might upset young viewers. Also the storyline is personal  because it has real life struggles. Therefore the film should be viewed by a mature audience. Lastly Iris contains messages that can apply to 15 years and over - the  film is about struggling with loved ones being diagnosed with cancer.

The difference between the certificates is the content within that age certificate. For example, A PG certificate is parental guidance and it's up to the parent if they want their child watches the film based on the content; it may upset a child. PG films are not made for young children, it's made for any age group. Adults can watch them too, e.g. The Artist.
But an 18 certificate would have to be viewed by someone 18 and over. This is because the content is unsuitable for anyone under that age, e.g. strong horror, this can disrupt and upset a person under 18, like a child - they be scared from the content. Although 18 and over are okay with it therefore the film is an 18.

• 
  Cinema release suitable for 12 years and over
• 
  Video release suitable for 12 years and over
• 

The trailer is PG 13 but the film rating is 15.

The tone coming from the trailer and the way it makes the audience feel can change the classification. For Iris it’s an unsettling tone which could disturb the audience therefore the film is 15. However the trailer is 13PG because the tone of it is not as disturbing. The trailer does feature drugs, any drug use will have to be PG13. Also there is alcohol abuse and depressing scenes which could influence a young audience making it unsuitable for children. And finally, the storyline is personal and it is a mature theme so it may upset a viewer.The trailer follows the 13PG rules.

PG-13 — Parents Strongly Cautioned. Some Material May Be Inappropriate For Children Under 13. A PG-13 rating is a sterner warning by the Rating Board to parents to determine whether their children under age 13 should view the motion picture, as some material might not be suited for them.

Up to parents to allow child under 13 can watch it.