I agree with the posts above, I really don't think that they place much emphasis on the BMAT at all. I seriously doubt a low score will impact whether you get an interview or not, especially if your other grades are good. I also seriously doubt that if you perform brilliantly at the interview they'll even bother taking the BMAT into account. I also think I'm right (but maybe am not) in saying that they don't really care about the essay bit of the BMAT and only really look at the scores on the first 2 sections.
To be honest, I personally think that the BMAT is the load of garbage and will give no accurate prediction over how well you will perform both during the medical course or indeed as a practicing doctor (6 or so year after you took it). A mate of mine for example said he did pretty badly on the BMAT, got like 12 or something, but has managed to get 'good' firsts in 1st and 2nd year. I mean he forgot to answer one of the 3 pathology essays this year and still got a first, which is pretty impressive.