I'm sure comprehensives reach a point where they are at a ceiling with results, which leads to frustration from teachers - they simply cannot teach any better, and they are getting the best they can out of the raw material they have to work with, whetehr that be 25%, 35% or 45%.
I know it seems like a cop-out, but we would first need to re-define what we mean by selection. For me it's not just dividing people into "academic" and "non-academic". the ideal system, plays to everyone's strengths, but that just isn't possible in the system we have with the limited resources there will inevitably be. I don't see the point, for example, of forcing soemone to do a GCSE in French when they are only going to get the lowest possible grade and it'll be meaningless for them, just so the school can scrape an extra meagre point on the league table. Because, let's be honest, that's why they do it - it's nothing to do with getting the best out of the kids.
All successful economies need a skills mix, and that inevitably means deciding, at some point, (I don't know when) that some people's skills lie in some areas and not in others. It means getting ourselves out of the mindset that academic=better and non-academic/vocational=sub-standard, and thinking in more pragmatic and less judgemental ways.
As I said before, if my hairdresser wanted to do A-Level French and go back to university, she has the chance in 2007 to do so, which she didn't in 1987.