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.

Similar films

*MENTAL IS A LIGHT HEARTED COMEDY HOWEVER I HAVE TAKEN THE MESSAGE FROM THIS FILM FOR MY THRILLER* 

Mental is a film released in 2012 directed by PJ Hogan and stars Toni Collette. The last film Toni Collette and PJ Hogan worked on together was Muriel's Wedding. This film was based on a real life event that the director PJ Hogan faced. Muriel's Wedding was about PJ Hogans sister and her wedding. While on the set of this film, Pj Hogan and Toni Collette were talking about Hogans nanny - a woman named Shaz. Hogans' dad had seen a woman waiting to go somewhere and decided to help her out and trusted her based on the fact that she had a dog. He then asked her to act as a nanny for his children because their mother had been taken into an institute.Hogan had said Shaz was the one person who helped his family through the time even t though "she was nuts". Within the first few days she was instantly able to better the girls' lives and towards the end she was taken in to the same institue as girls mother. So the young sisters broke in and helped Shaz to escape. Over the course Shaz helped the girls to see they did not have mental disorders but those around them did.

This is similar to my thriller with the aspect of the disorder. The main character in my thriller suffers from Dissociative Identity Disorder (DID). If the film was to continue past 3 minutes I would show one of the friends in of the main character swapping around the medication so that the condition would not be able to be controlled. Towards the end, it would represent something similar to the end of Mental. This would show the different sides to normality, the friend swapping the medicine would be seen as the troubled one and one character with DID would be seen as the perfectly "normal" person.

This here is a review of the film which explains the concept of the film:

Here is the trailer of the film.