I'm surprised by how many people are confused by what Comprehensive means.
My understanding is that schools are only truly Comprehensive when they exist in areas where there is no state school selective option at all. This means that the full range of ability in that area will be present in the school.
Traditionally (before 1970s) the top 25% ish in all areas went to Grammar and the rest to Secondary Moderns.
Secondary Modern is an unpopular term these days, having connotations of failure, a lack of academic exams and opportunity, so even in areas where everyone sits the 11* still, the non- Grammars are rarely refereed to as secondary moderns. They are often called Uppers, or in some areas comprehensives.
What complicates the issue is that where most Grammars exist, there isn't a full grammar system with everyone sitting the 11+. In some areas there are a couple of grammars which use distance as a qualifier as well as the exam, which means the places go to local children, but not everyone automatically sits the exam and a good number of high ability kids won't, meaning they are present in the non grammars....but these non grammars couldn't be called true Comps, due to the existence of the grammars nearby.
In some areas with the odd grammar and no distance criteria (super selectives) the places at grammar may be taken by kids from many many miles away. The effect is that the local non grammar schools may have over 90 or 95% of local children, so lots of very clever ones. These non grammars are closer to being comprehensives but again can't truly be called Comps due to the presence of the grammars.
Perhaps the people who are luckiest are those living in the borders of two counties - a grammar county and a non grammar. They sit the exam for the grammar and if they don't get in, they don't go to the secondary modern, but the comp in the neighbouring borough because they get in on distance. Of course, whether these Comps are truly Comps or the intake is distorted by the grammar over the border depends on whether the grammar takes in from outside the borough.
In the end, if there is a possibility to go to a selective state school, the non selectives in an area cannot be considered truly comprehensive. The level of diversity of ability in th non selectives varies wildly, determined by if the areas is a full 11+ area, if the grammars are superselective and of course by the affluence of the area.....it has often been said that the Uppers (secondary moderns) in affluent parts of Bucks perform really well and sometimes better than grammars in other parts of the country, due to parental support and involvement. Likewise, in Cheltenham, Balcarrass school which is non selective in an affluent a area does very well for children of a range of abilities. So it's not just selection that makes the difference, but as everyone knows, the affluence if the catchment too.