I'll be honest, I did 3 days of preparation for the BMAT. I did a section 2 and 3 each day and the marks I was getting turned out to be very close to what I got overall. (7.6, 5.6 and 4A)
If you're taking Biology, Maths and Chemistry for A Level, getting a 5 in paper 2 should be easy. If you don't do physics, just brush up on the basic concepts and formula and you'll be okay (I had to guess them all because I'm lazy and just refused to revise for physics bc I really dislike the subject to be quite honest, hence my lower score in paper 2.)
Paper 1, I think it down to a general knack for verbal reasoning and such skills they looked for in the UKCAT. I did about the same amount of prep for the UKCAT and got 730 avg. If you're generally good with critical thinking skills, finding flaws in arguments and conclusions etc. it won't be too difficult to get above a 5. If you're doing practices and aren't getting the scores then of course practice much more, but don't wear yourself out. All med schools give you the same job at the end of it, so trying to get into the competitive BMAT schools like UCL, Imperial and Oxbridge isn't all that important.
The essay - I did do quite a bit of practice for. I didn't write out essays, but I planned an answer for every essay question in the past papers to make sure I could argue for both sides. Ultimately, if you answer every sub point they ask you to, you'll get a 3 minimum. 4s and 5s come with original ideas, and an A should be alright if you got A/A* in English GCSE.
And if you think I'm some golden pupil talking myself down, I'm retaking 2 maths modules this year because I did pretty poorly!
Good luck