Most people who claim they live comfortably with multiple children on an income of 20k aren't accounting for the many benefits they receive. Often the rental support, childcare support, child tax elements, child benefit, all received tax free bring the household income to a much higher level.
DH and i are comfortable/well off. We have 250k a year or so income & 2 kids and are past the worst childcare bills.
We pay a lot in tax, and get in 12k a month after tax, ni, pension etc. £6,500 immediately goes out for mortgage, bills, kids activities, childcare. Around £500 of that is things we could cut easily - family gym membership & a weekly cleaner.
Of the other £5,500 - around £1,000 each goes on travel to work, coffees and lunches, meals out, buying gifts, cosmetics eg makeup, clothes and shoes including uniforms for children, charity giving. We drive old cars bought outright, mine is about 7 yrs old, DHs is a 56 plate, neither are high end brands.
£1,000 each is saved - our mortgage is only 3% atm and we need to save to reduce the balance before rates rise. Some of the money goes in accounts for the children.
The other £1,500 is what makes us feel comfortable/well off. It pays for holidays (not skiing etc -our summer holiday is a week in a self catered plus a week camping, all in uk, plus we have a week in spring or autumn in france or spain self catered) a new bathroom, when the carpets needed replacing, plants for the garden christmas, unexpected car repair bills or vet bills, house repairs eg we needed a drain unblocked recently & it was over £1k. sometimes its not spent and goes in savings.
If we chose to pay for private school, we would not feel comfortable. It would cost around £2,500 a month for 2 kids, so "comfortable" money would be gone, so would half the saving money. That would then have the knock on effect that the most of the other half of the savings money would go on higher mortgage rates when the fix ends.
We live in a 190sqm 1980s house with a modest garden in the south east, its a nice 4 bed but its not a mansion.
We are incredibly lucky but not rich beyond belief in terms of how we live.