MidlifeCrisis, and others that are baffled, I am sure that medschools would love to be able to rely on an aptitude test. However data suggests that none, apart from STEP for maths, are particularly good at predicting future performance.
UCAT and BMAT are filters as are A levels. Needed because too many qualified applicants are applying. Medical schools will know that some very good applicants will underperform at UCAT, perhaps because of a bad day, perhaps because they speak another language at home, perhaps because they are less good at speed tests, perhaps because they went to Welsh medium schools. Putting different weight on different filters allows medical schools to fish good applicants from slightly different pools, and perhaps identify applicants who are better suited to their teaching styles. And also encourage the more suitable applicants in their direction.
In practice a medicine degree is not that demanding for a science-y child. The academic bar is normally As, including chemistry, but beyond that, other than for the more academically focussed courses, having As does not give you much advantage. Other science degrees can be different. DD intercalated in engineering last year, and though her peers had similar, if not better academic credentials, some found even the pre-sessional course quite a shock. Not surprising given A, if not 2xA* is a normal prerequisite for an engineering course. (It was quite funny as DD found some students extraordinarily sniffy about being at "better" medical schools, until it became clear that they were not natural mathematicians.) Medicine instead, especially the more hands on courses, require a lot of complementary skills that engineers won't need to the same extent.
There will be some people lurking on this thread whose DC got poor UCAT scores. Mine was lucky as when she was applying Bristol used neither BMAT or UCAT, but at the time it was very worrying that despite a strong academic track record (she took 5 A levels and had strong predictions in all) and bags of volunteering, extra curricular, leadership and shadowing, her chance of being a doctor might be taken away. Simply because she is dyslexic with very slow processing. How is that fairer?
I would add that every year a few people message me for a handhold, as these threads often end up, like TSR, being simply for the overachieving 4 offer types. In the same way as I did to posters before me. (Waves at GoodbyeStranger, Peternas, AlreadyTaken.) I had a lovely message a few weeks back from someone whose son is about to go into his fourth year, and thriving, after one late interview which led to an offer. A poor UCAT is a pain and makes life more difficult, more difficult than it should, but not insuperable. I hope that medical schools start rowing back and start looking at the wider picture rather than increase their use of what is a very imperfect filter.