There are too many variables. Our income is far too low for most people to say we can afford private, but we have family help with one child, and pay for the other ourselves, but there are LOADS of things that we don't have that other families might - and do - consider to be absolutely necessary.
I know families who say they can't afford private who then turn around and buy second homes with cash and take £££ holidays, and also families who have a child in private and freelance. I know parents at school with lodgers to help make ends meet, people using family inheritances etc on fees instead of buying a bigger home or clearing the mortgage, people watching their peers upgrade and get bigger houses and fancier holidays around now while they stay in their "starter home" and go camping for a week in the summer. Other people might not consider any of that acceptable for their stage in life.
My husband goes a bit pale when the invoice comes in, but this is a temporary thing and I would much rather spend this money on education than hand it over to a bank as interest on a mortage for a 4th bedroom, you know?
But do assume you will pay out about 2k a year more than the advertised fees - there's always SOMETHING. A trip, sports stuff, music lessons, the deposit this year for next year's big trip. It's bloody endless. Just assume you won't have any money until after the GCSE's and then you can gently nudge towards a brilliant state 6th form.
(There are always great second hand uniform sales btw - less so as they get bigger and slow down, but a killing to be made when they're all still smallish and growing like weeds. Don't imagine you'll have to hand over £600 a year to the uniform supply shop or whoever. You'll only need to buy shoes new, really.)