I’d start by looking at the websites of the local private schools you’re looking at. They will have their fees listed somewhere on the website.
From that, you can work out how much of your disposable income you’d need for the fees. Add a bit extra for things like uniform, meals, extracurricular activities, school trips and so on. Private school fees also tend to increase annually above inflation, and there also tends to be jumps in fees at the end of key stages (so the fees for a Reception age child are typically significantly less than the fees for an A-level age child).
Your disposable income is whatever’s left after taxes, mortgage / rent, utility bills, food bills, insurance and so on.
That’s going to be different for everyone depending on their individual circumstances, so you need to think about what your own disposable income is like, not about whether someone else on the same earnings can afford private school for their DC.
Have a look at how the school fees added together compare to your disposable income, and that should give you a feel for how affordable it would be for you.