I was educated in the comprehensive system in one county and then moved to a grammar system in another (dad in army).
The comprehensive was far superior (but that's of course only my personal experience) plus you can have streaming in a comprehensive system it doesn't have to be mixed ability.
I started in the lowest stream in both schools and was able to progress to top stream but in all honesty in the top stream in the secondary modern I was unchallenged and bored. As I'd moved there in 4th year (don't know what that is under current system - first year of GCSEs) I wasn't able to attend the grammar.
A comprehensive system is not only fairer, it's more flexible and should be able to cater to all strengths and address all weaknesses.
Uni is not grammar for older kids, for starters uni students are adults. But currently the situation we have with uni is that the poor are put off by the idea of a mountain of debt with no reward. We're back to where only the wealthy can really afford uni.
As a pp said that's not just bad for those missing out personally, it's bad for the country. We need scientists, mathematicians, teachers, nurses, Drs, economists, physiotherapists, opticians, dentists, lawyers, engineers, architects, IT specialists, midwives, environmentalists, translators/interpreters, pharmacists, psychologists, social workers, even bankers, artists, media producers, journalists - from ALL backgrounds.
Actually I believe part of the reason we're in this mess is because we DON'T have enough people in some professions from anything lower than the upper middle class!
Diversity, different perspectives make us stronger, more adaptable and more successful!