Just reaching the end of this particular road, OP. XH and I are by no means rich, and we have had no help from grandparents etc. We were, however, determined to do it from Reception - Upper 6 come what may.
It's difficult to say what proportion of my income I'm spending on school fees ATM as my sector has been wiped out by lockdown and I haven't had any income at all for five months now. I haven't paid school fees since March as a result (school has agreed for me to pay nothing now, and smaller amounts over a longer period, once I have some money coming in again).
In a normal year, though, I would end up with about 20k pa (net), and spend £1,000 per month on one DC's school fees (I negotiated a deal with the school whereby I can pay monthly rather than termly). XH earns about 32k (net) and has also been paying about £1200 per month in school fees for two of our DC.
So that is a massive proportion of our respective incomes.
We have tiny houses, haven't been on holiday anywhere at all since the DC were pre-schoolers, don't go out, don't have takeaways or buy coffees/sandwiches, no gyms, no smartphones, read newspapers via month-long free online trial subscriptions, etc, etc, etc. We both consider those to be reasonable 'sacrifices'.
However, we have also had scholarships and, in one case, a huge bursary. I had a job at one of the DC's schools, which came with a fee reduction. So we have found other ways to reduce the fees (the amounts I quoted in the first paragraph are the amounts we have been paying for the two DC still remaining at school last year, which are both discounted prices).
Only one DC to get through sixth form, and then we will have managed it.
Neither of us has a mortgage, so that has also helped.
I believe 100% that it has been worth every single penny. One of my DC's schools in particular was so monstrously good that it's impossible to describe. We didn't do it just for the academic results (our local schools get excellent academic results). We did it for the whole package.