The scores for the UKCAT are calculated as follows:
In 2006, a cohort of candidates took the UKCAT. Their scores were calibrated so that the lowest was 300, the highest was 900 and the average was 600.
In subsequent years, everyone is compared to those pioneers and your mark is allocated in accordance to that. So let's just say for example taht the best guy for QR answered just 90% of the questions right, then he got 900 and anyone after that who answers 90% or more right will also get 900. That's why the mark cant be calculated because it is determined in relation to an earlier group.
For BMAT the situation is such taht most people will score between 4 and 6. I think only 5% or so of people get 7 or more. Oxford's website says that you should target a 6. The score is determined in relation to how many questions you get right but changes every year.
So for example, recently you needed to answer all but 3 questions right to get maximum mark. For section 1 you got a 6 if you answerer 21/35 questions right, and a 7 if you answered 25 out of 3.
For section 2, it was 14 and 18 respectively, out of 27.
Roughly speaking this makes it one mark for every 10% of answers correct.
For the essay this is marked out of 5, but 40% of candidates score 3.5 or more. Only 5% of candidates score 4.5 or 5.