In a big secondary, there are loads of bright kids to so the top set of a Comp caters to them
Depends on the cohort and how the school organises the classes.
In my comp, there were six forms in my year. But those six forms were split into two before setting, so there was a "top" maths group for forms A-C and then another "top" maths group for forms D-F. So basically a top, middle and bottom group for three forms and another top, middle and bottom group for the other 3 forms. I was the top group for D-F and we had a massive range of abilities, between A and D in fact! My comp certainly didn't "cater for" the brightest - you simply can't when you have such a large range in a single class. The brightest were basically ignored as the teacher concentrated on getting the D graders up to a C!
The middle groups were likewise two groups of wide ability, in fact, worse because half the groups were doing foundation and the other half higher, so you had two groups where the teacher was splitting their time and lessons between two different exams!
The "top" group should have been out of all six forms and then it would probably have been an A-B group, with the second group being the C-D group. Then you could have had the top two doing higher level and the bottom 4 doing the foundation level. Then each group would have got the proper teaching to their level and no poor sod of a teacher would have to try to teach both foundation and higher level students in the same class at the same time!
For comps to work, they need the setting to be at a fine enough level. Stupid having 2 groups of A-D students as it's simply too wide. Just like stupid having 2 middle groups, both of whom had some foundation and some higher level students.
If we're going to move forward with just comps, then they need to find ways of properly teaching ALL levels to their abilities. Too much emphasis is on those at lower levels and the "quiet" brighter ones are basically ignored. These days, the way to get attention is to be just below some threshold or other so the school put all their efforts into getting you over that threshold. Kids who are working to grades B-C at the moment but capable of A/A* with a bit of push aren't getting that little bit extra support and that's just plain wrong and is where the selective schools have the ethos to encourage that "push".