BBFC

U:A film classified as a U is suitable for all audiences aged 4 and over (although it is impossible to predict what may upset a particular child). The film should be set within a positive moral framework.

PG:Films rated PG (parental guidance) can be viewed by anyone and should not unsettle a child aged 8 or over, it is up to the parents to decide if they feel the film is suitable for their child to view. No theme will be included that is inappropriate for a child although there could be a possibility for mild language and violence.  

12A:Is used in cinemas to classify films. No one under the age of 12 can view a 12A unless with adult supervision. Although younger viewers can go and watch these films it is suggested that the film is suitable for those around the age of 12 and above. Moderate violence, discrete and brief sexual references, and strong language (depending on the manner of which it is used) may be included in the content of a 12A film.

12: Films classified as a 12 are allowed the same content as a 12A. 12 is used to classify videos works so no one under the age of 12 can buy or rent a film classified as 12.

15:15's can be viewed by audiences of 15 years and older. No theme is prohibited  providing that it is appropriate for the viewing of those aged 15. 15's may include strong violence, frequent strong language, portrayals of sexual activity, drug taking, strong verbal references to sex, sexual nudity, brief scenes of sexual violence or verbal references to sexual violence, discriminatory language or behavior.

18:Films classified as an 18 are to be viewed by those who are 18 and over. There is no limitation on what can be shown in an 18 film and no theme is prohibited, it is up to the adult what entertainment they would like to watch.

Typical Classification for a Thriller:

Most Thriller films are often classified as a 15. This is because 15's allow strong language, strong violence, sex references, hard drugs and is allowed to show a limited amount of blood and gore from injuries and all of these aspects are what is included in a lot of Thrillers.   
However there are exceptions where some are classified as a 12 or 18. A Thriller rated a 12 would have mild language and possible minimal use of strong language if justified, as well as minimal detail on violence and reference to sex. A film classified as an 18 can have pretty much anything included, it would be a lot more brutal and gritty than a 15.