Quite a few institutions across the UK struggle to recruit enough students with adequate grades (from the main secondary school exams) to make their degree courses viable, so for them there is no real issue of selection - their problem is simply finding enough 'bodies' to occupy the available places. But for many degree courses at many institutions, there is real competition for places, and so departments do have to make choices regarding the sorts of offer they make to potential students. Places can be, and often are, rationed by setting a higher entry standard than the normal minimum qualification. Some institutions interview candidates to try to assess their academic potential; a few even set their own entrance examinations if they consider that the normal school examinations do not bring students quite up to the right level. This latter practice is most common among the leading universities offering degrees in mathematics (see below).
Now, up to this point the story is, I think, quite uncontroversial. It gets trickier, though, when we dig down a bit and consider the practicalities of the selection process, something that has become especially politicised of late in England. The issue boils down to this:
(a) Should universities insist on whatever entry standards they think appropriate, while supporting potential students from disadvantaged backgrounds (meaning either poor school or poor social conditions) to bring themselves up to the required standard through various forms of outreach - such as summer schools, short courses, school visits and the like? Or
(b) Should they make special efforts to accept additional students from disadvantaged backgrounds, even if that might mean making some offers to people who are less well qualified academically than others (presumably from 'better' backgrounds) whom they might otherwise have accepted?
Personally, I have always strongly favoured option (a) above, and have never greatly liked the sort of social engineering that seems to be involved in (b) (and I don't mean to sound shockingly elitist when I say that). Of course, universities have always made judgments about student potential, that's quite normal, but I do think students have to have the right qualifications - at the right level - to do a course, otherwise they will either fail or drop out. There is actually quite a high social cost associated with taking on students who are not really ready to undertake a particular course. The other way of mitigating these costs to some extent is through dumbing down courses, making them somewhat less demanding; and through providing lots of extra support to the weaker students. I really wonder whether this is the best route to take, given the highly competitive world we live in.
Given all this, the still on going controversy over whom to appoint as the new head of Offa (Office for Fair Access), and how he might choose to use his powers, seems to me completely counter-productive (see Times Higher Education (article by Simon Baker), and The Times (article by Camilla Cavendish), both today). For one thing, it's unfortunate that this new office has become so political, even before it really gets started - it was supposed to be independent of government, but how can it be now?. For another, although Offa was part of the package of measures that included allowing higher fees at English universities, I think it was a seriously bad idea. Personally, I would just abolish it, as there are already plenty of channels through which competitive pressures of all sorts are brought to bear on the universities.
Thinking about point (a) above, and mathematics, how do we bring potential students up to the very highest standards so they can succeed at the best universities? The reality of life, it seems, is that even quite good secondary schools in and around Edinburgh, for instance, lack the teaching resources (and sometimes, perhaps, the specialist ability) to provide extra tuition to the one or two students per year who might be good enough to try for Oxford or Cambridge entrance in mathematics. Almost by accident, I've gradually found myself acting as tutor to a scattered group of these students, now from three different schools. The students are very bright, and they work hard, but they do need the extra support - which none of their schools or families can provide themselves. So advanced maths tuition has become a sort of retirement 'hobby', if I can put it that way, which I do entirely for the satisfaction it offers, and for the fun of trying to solve the problems I set 'my' students. Getting students well prepared to cope with high standards is surely the right way to go.