Like you, I'm a parent and I agree that my DD's school seems very aware of the impact of rising fees on parents and seems to try to keep fees rises as low as they can.
Personally, I don't think it's necessarily the right thing for schools to reduce their educational offering by 20% in order to keep the overall cost to parents the same.
And it would be reducing their educational offering: most of the fees go on staff salaries, and reducing staff would inevitably mean they couldn't offer as much.
For me: I'm currently paying the government an extra £13k income tax in order to be able to pay the school £18k. So I'm paying £31k (gross) for DD to get £11k more education than she would in state (assuming per student cost of £7k in state).
If the school reduce the education they're providing to £15k (so that with VAT it comes to £18k), then I'm still paying £31k (gross), but DD is only getting £8k of extra education for it than she would in state.
It's gone from DD getting 1/3 of what I'm paying, to her getting only 1/4 of what I'm paying. The rest goes to the government.
I think if private schools reduce their offering, the value for money quickly becomes very poor.
It's because we're giving up the state school place - which is a large fixed amount (average 7k). And we're paying 5k income tax to earn the 7k of education which we're paying for ourselves as a direct replacement for that state school place we've given up. So that's 12k fixed cost before we can even start getting any additional education for our children. Of course we must also pay income tax on the money we earn to pay for the additional education as well - but at least that's proportional to the education we're adding.
Do you see how the 12k (gross) fixed cost we incur to replace state education before we can even start adding any extra educational value means that the closer private school spending gets to state schools, the worse value it becomes? Hope it makes sense!
(Of course, for some schools, it might be the right thing to do - if the value they're giving is still worth it to those parents. But it will depend on the school).