I have several times taught children in a manistream, comprehensive entry prtimary who as a school we would have advised should attend a Special School instead, and have also rtaught children who we would have recommended a specialised unit attached to a (different) primary.
In all cases, the parents chose not to follow that advice (advice also given by all professionals working with the child, including advisory teachers, paediatricians etc) so we could not refuse admission.
A grammar school can refuse admission to any child, SEN or not, who fails the 11+, regardless of the views of the parent. That is the difference between the two.
That does NOT mean that all comprehensive schools have identical cohorts (as any school would know, even for comprehensive schools serving an identical area each year, the ability of each year's coort can vary dramatically)
As I said before:
"A comprehensive school is a comprehensive school if its intake matches its applicants because there are no selection barriers other than distance / catchment to pupils being admitted, and if no other state school preferentially admits selected pupils from the school's effective catchment.
So a school is not comprehensive if it selects some pupils by aptitude, any pupils by faith, or by ability, or has any significant number of potential pupils drawn away by nearby selective schools.
Comprehensive schools WILL vary in their composition, because they are situated in different areas and parents may exercise their preference to apply yo another school. The school remains comprehensive unless the alternative school selects any of those applicants by ability or aptitude or faith."