Trailers/advertisements

Audience actively choose to see a full length feature based on expectations of the particular genre at the given classification (including Consumer Advice and Extended Classification Information)
But, they have no choice about which trailers can be shown and may be completely different to the tone and content of the film that they are watching. 
Trailers which are borderline material are less likely to be justified by context and more likely to cause offence based on that fact that they're short and self-contained. 
Therefore, when classifying promotional material, they need to be more restrictive than for equivalent material in a main feature i.e. 

• strong language is not permitted in trailers at any category below 15.
• only one use of strong language is permitted in a trailer at 15 and must not be neither threatening nor aggressive. 

Advertisements including alcohol of any type can be passed at any category. 

But, there is a more restrictive approach if the alcohol is glamorized or shows a successful lifestyle. 

BBFC not responsible for the exhibition of cinema trailers, including alcohol and has no involvement in deciding which films they appear before. This is the process of the cinema.