OP, I'm not quite sure what to say (have NC for this).
I have two DC. Disclaimer is that I am absolutely committed to independent education come Hell or high water. I thought about this before I had them (I have no experience of state schools in my family, and XH has none in his, so there is a degree of ignorance at play too).
We live in an area with decent state schools.
Anyway.
DC1 is at a boarding school where the full fees are now over 40K pa (and would be worth every penny, if we could afford it). Fortunately he has a scholarship and bursary, so costs about £1,300K per month including extras (XH pays this).
DC2 is at a day school, where the fees are about £16K pa. XH and I share this, with me paying the bulk (£1,200 per month).
I earn about 30K pa. XH about 40K. But, having started the DC on this route, we are both absolutely committed to seeing them through (they are now Year 11 and Year 13, so we have almost got there). I don't know whether you would call this an 'average' income, but I'm giving you an idea.
Uniform is all second-hand. I don't remember ever buying anything new. So it's a minimal cost. Fortunately most people do the same, even if they could afford to do otherwise. Weirdly, there's a bit of a thing at independent schools that the only people who buy new uniform are people with either only one child, or people with money to chuck away.
That's the positive side.
The negative side is that you would have to be prepared to do without absolutely everything. No holidays, no new clothes that don't come from a charity shop, no holiday activities. I have a car, on a PCP scheme (covered by my Child Benefit payments). I am happy to do this, because my DC's schools are brilliant and I am determined to give them the experience that I had, come what may. However, they have to put up with rather boring school holidays as a result. We live in an eternal renovation project which I can't afford to finish. I hang around supermarkets at discount time to save money. We don't do school trips etc as we can't afford them. But I am happy to suck that up.
It would be lovely to have it all ways, and pay the fees without thinking, and go on holiday and send them on school trips. But something has to give, unless you are super-rich.