Research & Planning: Regulation & Censorship

For my A2 media work, I am considering which certificate to use for my film.
I am thinking of making the certificate either a 15-18 as this is expected of the horror genre. Most films that are a horror genre are usually given the 15-18 certificate,
However, I have to take into consideration that my trailer could be given a lower certificate than what the film has been certified as most horror trailers are like this.