Median UK household income is about £29,000 per annum
Median number of children per woman in the UK 1.8
Median private secondary day school fees in the UK £17,000
Average private schooling family is not "just ordinary".
They may have been to a slightly greater degree 30+ years ago, but fees have risen massively in relation to incomes. Even 30 years ago nobody at my not very special private school lived in a council house, even if perhaps in a few cases their grandparents had done. The few children on scholarships were still usually from lower middle class families and often actually had at least one parent who'd been to private or grammar school. In the 80s there were lots of upper middle class "professional" families and some wealthier farmers, but private school is no longer in reach of so many of those families unless they only have one child and/ or grandparents are paying.
The "just ordinary" argument is pretty disingenuous, although I'm also not really sure why parents paying fees are so determined to argue that they and their fellow fee paying parents are so ordinary... Is it some kind of humble brag? A back handed way of claiming that they just live their children more?
Very few people eat value dry pasta and tinned tomatoes every night, don't buy a new piece of clothing in a decade, cut their own hair, have no pets and walk everywhere in order to send two children to private secondary school, but it wouldn't be possible on an average household income with an average family size otherwise...