I think the general point, which a number of people have made, that it depends on primarily on local circumstances, still applies. I also think Maria and Nanjizal make good points, but I would question whether this is really a terrain for moral absolutes. I used to think it was, too.
As far as financial exclusion goes, our local prep costs around £5k - £6k/year, including school meals and a range of extracurricular (sport, music, languages) that it would cost a small fortune to self fund. It is not financially out of reach to families on a lot less than £100k/year, but it does mean sacrifices (e.g. no or fewer holidays, an older car, etc) that I can totally see why parents would choose not to make, especially if they're happy with their local state schools. If they are on average or beneath average incomes, there is no choice. And I can see why that feels wrong.
But here's another side to it, and why I now think twice before judging parents who don't go for the state option, as we did. Local state schools have little in the way of ethnic diversity, whereas the prep has about a third of children from ethnic minority backgrounds. I can see why some parents might feel uncomfortable sending their children to schools where 98% of the children are white. (I'm also aware of the irony that the situation is often the other way around, and prep schools don't reflect the diversity of the communities in which they are based, either socio-economically or ethnically.) Of course, there's then the chicken and egg problem that local state schools will stay homogenous as long as this dynamic continues.
In the end, I can see how it is better to work with other parents to improve local state provision than to give up and look to the private sector, but I can also see how it's often local circumstances that often drive parents' decisions, and how I'm not in the position to judge them.