Deciding the rating for our thriller

We as group decided after looking at the details of the U.K rating system that our thriller would come under the '15' bracket.
We though this was best mostly due to the constraints put on the '12a' horror. Our thriller would have some more disturbing and sadistic scenes involving the antagonist than what is deemed acceptable under the '12a' bracket of rating. Also the '12a' bracket does not allow any great detail on the level of injury or a great deal of blood in terms of this injury. The '15' bracket allows more freedom in these fields. As a group we did not see our piece as being extreme in any way to be deemed as in the '18 bracket as these thrillers would have extended sequances of disturbing horror and gore filled and exageratted violence.