I have an ND child who is at a fee-paying school, and I don't think it's an justification for the existence of fee-paying education - that's clearly wrong way of looking at it.
In choosing a fee-paying school, I knew I was using money to get something for my child that they might not be able to get elsewhere. I also knew that plenty of ND children wouldn't be able to get that extra support in the state sector and that is pretty hideously unfair.
I also completely take the point that, by using money to secure an advantage, that is one less middle class voice in the state sector directly invested in, and arguing for investment in state education - including for ND children who desperately need it.
However, when it came down to it, I did something as a parent that was instinctive rather than principled. I made a choice that I thought I was best for my own child (not society at large) because I was frightened of what would happen if I didn't. It's not any more complicated than that.
In the end, in my own set of family circumstances, the choice made an enormous difference in a way I couldn't have envisaged and I am so, so grateful for that. I recognise my privilege but if I had to make the choice 100 times over, I would make the same choice every time.
For the avoidance of doubt, I think every ND child should have access to what mine had (free of charge) and I am very sorry they don't. However, it's because of experiences like mine, that you find many passionate defenders of fee-paying schools in these types of Mumsnet thread - especially ones obsessing over teens around results day as if exam results are central to the private v state school debate - which for parents like me is to largely miss the point.