BBFC

Considering the content of my thriller and the guidelines provided by the British Board of Film Classification I would say my thriller would possibly be rated at 15.
These are the characteristics of 15 from their official website:

How much strong language is allowed in a 15?

There could potentially be a great deal. At 15 there is no upper limit on the number of uses of strong language (e.g. ‘f***’).
Occasionally there may be uses of the strongest terms (e.g. 'c***'), depending on the manner in which they are used, who is using the language, its frequency and any special contextual justification. However, continued or aggressive use will not normally be passed 15.

What about discriminatory or offensive terms?

There may be racist, homophobic or other discriminatory language, and the work could explore themes relating to this.
However, at 15 the work as a whole must not endorse discriminatory language or behaviour.

How much sex and nudity is allowed at 15?

At 15 sexual activity can be portrayed, but usually without strong detail. Some sex scenes can be quite long at this category.

Though nudity may be allowed in a sexual context there will usually be no strong detail. There are no constraints on nudity in a non-sexual or educational context.

There can be strong references to sex and sexual behaviour, but especially strong or crude references are unlikely to be acceptable unless justified by context.

Sex and sex references are treated the same irrespective of sexuality

Can there be strong violence?

Yes, at 15 violence may be strong. It should not dwell on the infliction of pain or injury, however, and the strongest gory images are unlikely to be acceptable.
Strong sadistic violence is also unlikely to be acceptable.

What about sexual violence?

There may be detailed verbal references to sexual violence (for example descriptions of rape or sexual assault in a courtroom scene or in victim testimony) but any portrayal depiction of sexual violence must be discreet and justified by context.