There is selection anyway, either within the school (sets) or with different schools (private selective or grammars).
I would not like to see grammars introduced in Hampshire, as there are many good comps, and I like the idea that a child (including my own) can start of a bit lower (esp. boys often do), and the gain momentum later on. It's nice if they are in an environment that does not put a ceiling on their achievements.
Saying that, some comps work with very rigid sets, rigid targets and predicted grades, so you can be "stuck" in a low set, with low expectations.
The problem with the comprehensive system is that not all comprehensives are good in this respect.
As schools vary so hugely, it is hard to generalise.
I'd think I'd favour a comp system which allows for movement between sets, and for sets not to be all linked (in some comps if you are bad at English, you get in bottom set for all humanities and languages).
I was lucky enough to find one like this.
But some people prefer rigid sets (usually parents of top setters). So they should have that choice.
I am grateful we don't have a grammar school here to fuck up the system (as the remaining schools would be "second best" by default), but I guess I don't oppose them. If that's what people want, give it to them.