Hostess, but surely your DD's job was the alternative to a fourth A level.
DDs sports commitments probably made a job impossible, but it was perfectly possible to fit in studying. (I think till year 11, she managed all her homework on the tube.) This would not apply to your DD, but mine was staggered at fellow students who claim to have worked so so hard at A level, without job, obvious extra-curricular etc. A University workload then becomes a real step up.
I agree with Stranger about a fourth A level allowing breadth. You may not know when the analytical and essay writing skills that come with history will be useful, but they will. (It almost certainly helped non medic but mathematical son ace the language section of the graduate admissions test/GMAT, even though for him it was by far his most challenging A level.) Similarly my guess is that the British med student on some sort of placement who helped my daughter when she was taken down a mountain on a blood wagen, kept French up to A level. Whilst for DD it helped keep the doors to alternative, more mathematical, degrees open.
I think Fergus the Frog's advice is excellent. I would perhaps add that it is worth looking at placements, and how varied they are. Bristol, where DD is, has a really good mix of urban and rural, large hospitals and small, and everything in between. A car is not essential, but it is a huge help.
The problem with UKCAT is that it is a timed test, so not great for bright kids who are not fast. It is also a long test, so dyslexics, like DD, are likely to find that extra time does not help as they are brain dead before it kicks in. Practice does seem to make perfect, and a boy we knew who practiced for an hour every day for six months, came in the top 2%, and gained 4 offers, yet his subsequent progress does not suggest he is naturally top of the year.
FWIW DDs very selective private school seemed to assume people took both. Take UKCAT early and if you get a high score you have more options, if not, or if you want to go to Oxbridge/UCL/Imperial or another BMAT school, work like blazes for the BMAT. DD was warned that a couple of strong A* applicants each year failed to get places, and this was certainly true in her year. One got a place at a very competitive med school on reapplication, whilst the other changed their mind about what they wanted to do. Despite MN views on private school spoonfeeding, I was not aware of things like UKCAT practice. The help was probably one step removed., as in you are bright kids, this is what you need to research, now go away and do it.
DD got school comments on her PS, but it remained quite individual and slightly angular. But then, they can't expect a very polished PS from a dyslexic. It was also shown to a rural doctor friend who mentored local state medicine applicants, to check that nothing was obviously missing.
This is the list of intercalated degrees available in the UK www.intercalate.co.uk/ There are some really interesting options, that will give real depth to understanding for people who want to, say, specialise in paediatrics; women's health, emergency medicine, radiology, ethics, sport etc. A course which allows you to study at elsewhere at a national centre of expertise is worth giving priority to.