LesleyA,
Probably the best way to think about it is by first imagining the full range of children's abilities.
There will be a small percentage of children whose disabilities are so great that they will attend specific 'Special schools' that usually specialise in certain disabilities.
A comprehensive secondary school will then take all other children who live within their local area. They will therefore cater for children of every ability - the range will depend somewhat on the school's popularity and the demographics of the local area, but in principle they will contain children of all abilities from the very highest to nearly the lowest. They therefore offer a range of subjects and qualifications that are suitable for all of these children. A proportion of these students will go on to university, because they will get the grades in academic exams at 18 that give them the 'entry ticket' for these.
In a grammar school area, the grammar schools will select children towards the higher end of the ability spectrum. A higher proportion will therefore go on to university, but that will be a very similar proportion of children of like ability at comprehensives. (So for example, if 50 higher ability children at comprehensives and grammar schools are compared, a very similar proportion of these children of matched ability from each school will go on to university).
The other schools in grammar areas are technically secondary modern schools, though some are misleadingly called 'comprehensives', or 'high schools' or just 'schools'. They will contain fewer high ability children, but if they get the required grades at 18, there is nothing at all to stop the children from these schools from going to university as well.
In England, entry to university is only on the basis of exams taken at 18, and these exams are fairly universal across all schools (most are A-levels, though there are others). Entrance to university is on the basis of A-level results, so any pupil from any school (or e.g. home educated) can go to university if they get the required grades.
Oxford and Cambridge, and applications for medicine-type courses, are the only exceptions, as they have specific additional entry exams in general, though again attendance at particular school types does not prevent any child from applying to these universities and taking these exams.