You are missing my point.
Costs have gone up, state sector spending. Those 'same provisions' have been cut in the state sector. They don't have capital, investments, donations etc to absorb the rise.
Wages have not risen in line with inflation.
Private schools could have reduced provision to keep them affordable to the children with SEN who desperately need those smaller class sizes.
They could offer a few less extra-curricular activities to keep the costs down and make it affordable to those who need it.
But that's not what it's for is it.
I'd argue that you only care about people not affording it when you're the one not affording it.
To declare my interest, my son goes to a state grammar and loves it. He is doing very well. We could afford a tutor for 12 months plus theatre trips and national trust membership and all the other middle class privileges. He benefits from quiet classes and children who want to learn.
But i can't defend it. He benefits from an existing system whilst his mates at the secondary modern are put at a disadvantage. It's not fair, but it's in our favour
If labour suddenly announced they were getting rid of them it would harm us, but i couldn't tie myself in knots to claim it was wrong or unfair.