One of the differences between 'comps with setting' and grammar schools is the flexibility.
In my DC'c comp, there are children who are, for example, in top set Maths and a very low English set, and vice versa. Children who are top set Science but middle set for humanities. Top set PE but bottom sets for academic subjects - and vice versa. Higher set languages, lower set Art (looking at you, DS).
The setting also occurs gradually - Maths in Year 7, a couple of other subjects in Y8, more in Y9.
It is also perhaps worth noting that ALL our local grammars have sets as well..... but I do know that at least one or two who have got into Grammars on the (traditionally VR-based, though that has been replaced) test and then found to really struggle in Maths have not been provided for, because no set low enough was available, and then not been allowed to take GCSE Maths at that school, because it affects their 100% pass rate.....