They can't go moaning about maths A-level being inadequate for university study when they don't require further maths.
As you said, the top universities do insist on further maths - which excludes pupils who were erroneously told that it was fine not to take it. It is also harder to get offers from Oxbridge for maths without having physics. Despite being well publicised, this is another issue that holds back pupils from weaker schools.
Other universities don't actually go around moaning about maths A level being inadequate for STEM - they just deal with it. But in reality the drop out rate is a bit higher amongst those who don't have further maths at places like Bristol. So even if students get in, they are at a disadvantage by not having it. (If they get through the first year, they don't do worse afterwards but they are more likely to drop out of some courses if they don't have FM.) This is made clear to students during admissions days - if they have the option of self-studying FM modules or studying them at sixth form they are strongly encouraged to do so.
And as you say even top universities are actually being forced to take students they would prefer not to take - otherwise they will go bankrupt, with costs increasing, but fees fixed and research income being pushed down. (We spend about half as much of our GDP on research as the rest of Europe and North America.) The biggest problem is amongst STEM students who didn't do A level maths (biological sciences etc) and whose maths level is truly shocking. I know RG universities, for example, have to start their maths for biologists courses with fractions and decimals...
BTW the draft A level maths curriculum was in part the responsibility of top RG maths departments who are utterly fed up about the declining standards of A level maths... As I said upthread, I do have deep concerns about the implications of the new A level for weaker students, but something does have to change at the top end.