So this is interesting - as always, private vs state turns into a bit of a bun fight!!
We have enrolled my DD at a pre prep for nursery, to get a look at what prep life offers from the inside, but we are still planning to go to the open evenings for the state schools, because we are not decided yet. For us prep school would be c.20% net income (our other outgoings are not big).
We live in a grammar school area, so we have a shot at good grammar schools at secondary and have an extremely good independent (amazing results), plus lots of other independents which are good at some things, less at others, suit different types of kids. If I had a bright child who had a bad day on the grammar school test, I'd probably pay to avoid the local academy, as in my town a lot of the brightest go to grammar and then a lot of those who just missed out go to the faith a school, so the cohort doesn't have many high achievers there and I have heard behaviour is mixed.
I went to state primary, then to a comprehensive school, then on to a sixth form that was attached to a much higher performing school that kicked out kids who didn't get 5 a-c and a B in the subject they wanted to do A levels in, so it was like a grammar sixth form really - I couldn't get in in the lower years as it was a faith school and my parents didn't put the effort in to "find god" (my mum now says she wishes she had!). I went on to oxbridge. You could hold me up as an example of "well she didn't need any more than a bog standard comp", but my experience in the comprehensive (which I understand is not reflective of every comprehensive) was absolutely dire - culture of bullying, particularly of bright kids, not cool to try etc. Even some of the teachers were a bit like that - they certainly made no effort to try to protect the bright kids from it. I taught myself 3 of my 11 GCSEs entirely as the teaching was so poor. I channelled the anxiety I felt all the time about where the next set of verbal abuse was going to come from into an eating disorder, which took a long time to recover from. The state sixth form was amazing - there were about 30 of us out of 120 with the kinds of GCSE results to get into oxbridge and everyone basically got into a good university for their choice of subject, whatever that was. So my views are that I would absolutely send my child to the school I attended for sixth form, but I would try to avoid sending my child to any school where she would stick out like a sore thumb and run the risk of bullying for working hard. Academically, though, I absolutely would have been fine anywhere - I could probably have taught myself all my GCSEs from home with a few books if I had to, as I was motivated and am very good at learning from a book - and I did loads of extracurricular sporty things outside school, played basketball and netball and ran for my county. At Oxford, my college was quite dominated by public school confident types and some were very sheltered, some very much not (drugs, wild travel, exposure to things that I certainly hadn't seen in my Home Counties childhood!) - I certainly felt initially that I had more in common with the state school kids, but that wore off a bit, though my very best friends from Oxford are state educated.
My DH is a product of prep and public school. He has been brought up with "private school confers massive advantage" because that is of course what you have to believe when you pay for it. Now he is married to me he realises that we got to exactly the same place, so he does question that somewhat! He was absolutely miserable during his common entrance exams (for which he was put under a lot of pressure) and at his single sex boys public school.
It is fair to say that we are both looking for an environment for our DD that is different from what each of us had at points in our school careers. I don't know what is right for us, but the reality for us is that it is about finding the right school rather than the right sector and I realise that this makes us extremely privileged to have the option to pay if we feel we need to- my parents didn't have this choice.