Our joint income varies a lot but has been £300-600k in the past few years. We have one dd who has just started at prep school and we plan to send our younger dd there. I don't think we would have considered private school if our incomes were below £150k - it would have made more sense financially to move to the catchment of excellent state schools. For us, the prep offers more of the things we value than the best state primary we could have moved near - class sizes of 10 in reception (with full time TA), sport every day, excellent language teaching, creative arts and extracurricular activities, and of course prep for 11+. It's also nice not to worry about ferrying them around to after-school activities or exam tuition. Fees are £17k a year and we pay for uniform and after-school clubs on top but most other costs are included, including lunch and day trips. We are in central London. Looking at the senior day schools where DD would most likely go, they are about £23k a year although I'm aware they will be far higher by the time she goes.
We don't have any help from grandparents or inheritance - everything is paid for out of income, although we have significant assets which we don't touch but help us feel secure (saved from earned income and grown through shrewd investments). Most parents we've met at the school so far seem to work in law or finance, but there's a wide range of jobs (but all very senior). We only live in a small flat right now which is mortgage-free but upsizing to a 4 bed terraced house with a hefty mortgage soon. Hardly anyone drives to school so I have no idea what cars they own (we don't own one at all and take the tube). There are bursaries although I don't know anything about them, and we've used the second hand uniform sale although we could afford to pay for new, but I don't feel there's any need to.
We feel we can comfortably afford the fees without much impact on our lifestyle. We have overseas holidays (but not luxury resorts and mostly short haul), pay for whatever days out we like (lots of family theatre and concerts, theme parks), pay for any extracurricular activities the dds like, buy clothes and treats when we want to (which isn't that often), we don't budget with everyday spending but we're not really big spenders. No state grammars nearby and I enjoy our city lifestyle too much to move out to the outer zones to live near any of the London grammars (and I'd never want the dds to do a long commute to travel to them).
In our London borough, almost 40% of secondary-aged children go to private school so it's not a small minority. But it's a borough with some of the most expensive properties in the country (although it also has a lot of deprivation too).