I am 'a true believer in comprehensive education', but I don't want mixed ability teachinbg in all subjects.
The poiint about a comprehnsive education - all abilities under a single roof - is the flexibility it offers for every child to be placed in the correct group for their ability, subject by subject. A child who excels at maths but struggles in English? Great - top group maths, low group English are both available. Excels at music, art and design, finds PE hard, middling at English? Again, top set for some subjects, not for others. Mixed ability teaching for some subjects, where it works or where ability is not easily measured on entry to the school: my DS's comp sets very little in year 7., much more in Year 8, and almost totally from then on, because in subjects like Design, Art, Music, PE, MFL there is no good data on which to base subject-by-subject setting until after the child has been taught that subject in a secondary setting for some time.
What I have against education that is segregated by a crude measure of 'ability' at a fixed age [whether VR, NVR, or a test of maths and english as well, it is a really crude measure] is that it denies access to correct groups for many children. A child who is level 7 maths but Level 3/4 English will fail the 11+, and therefore be unable to access the true 'top set' in Maths which is their educationally correct place. Equally, the hundreds of children who are essentially identical in ability - those 4 or 5 marks each side of the pass mark, who could on another day have been swapped over with absolutely no effect on the average ability of the cohort in each school - have different access to e.g. separate sciences, vocational courses etc.
There is no educational justification for the child 1 mark above and the child 1 mark below the pass mark receiving wholly different educations under se[arate roofs - their educational needs are so similar that they SHOULD be co-located, with all the flexibility over time that that allows.