Hey there, there isn't really a minimum score, I guess the emphasis that Unis place on it varies depending on how strong the rest of your application is etc. But, like lil_groovy_dude said, try and get 19!!
I know for a fact for a fact that Imperial don't focus much on the BMAT (it's also the first year they've been using it) since I got 16.4 and was interviewed.
However, I have heard that some Oxford applicants who were rejected w/o interview and who had strong GCSE results and A-level predicted grades were told that their BMAT score was one of the main things that let them down when recieving feedback.