If your DC is at a comp I would look and see how many A grades it gets per year in the STEM subjects at A level.
When the science A levels got tougher a couple of years ago, some comps really struggled to provide adequate preparation.
If you see just a sprinkling of A grades it's a sign that extra tutoring might be necessary.
Everything hinges on the predicted and then (in a normal year) the grades achieved in the exams.
Candidates with brilliant UCAT/BMAT, volunteering, references, extra curricular accomplishments, and holding offers, get rejected every year for the want of perhaps one or two marks on just one of the three A levels. It really is viciously competitive.
And when my DS got offered a place there were a few candidates from European independent schools at the MMI stage. I'm not sure if this has changed due to the loans situation changing post Brexit, but it did make him appreciate the breadth and depth of the other candidates' qualities, many of whom were incredibly well rounded and multi-lingual.
So I would prioritise A levels, as these are the variables that can be most affected by the lack of adequate groundwork.
There really is no substitute for getting the high grades. Achieving a contextual ABB sounds easy. But the exams can throw up the unexpected for the poorly prepared.
And getting something done about poor teaching can be harder than it sounds in the state sector. By the time parents realise that something is wrong, the pupil could be half way through sixth form. Then who wants to be the one to complain when the judgement of teachers is more important than ever.
I do empathise with new entrants. The whole process gets more and more onerous each year. Who would believe that you could waltz in to med school with classics and the odd Grade C, a few decades ago!