@JustBecauseIcanComment
I’d also add that those who can’t afford the VAT are those that will move to state, at a cost to the state of £8k per child. So you have different groups of people
a) those paying the highest fees and who are more likely to have been able to access funds to pre-pay fees. The government receives no VAT on these fees, but the input VAT can be reclaimed. Boarding fees can exceed £60k a year. The government had assumed they’ll be receiving £10k VAT on this. In reality, a lot of families who could afford £50k will have pre-paid. This hasn’t been accounted for in the IFS figures.
b) Those paying fees whose wider family have stepped in to help. Grandparents paying fees avoid IHT. Some will have pre-paid (avoiding VAT altogether). Some will be paying going forward, paying 20% VAT but saving 40% IHT. This hasn’t been accounted for in the IFS figures.
c) families who’ll carry on paying regardless. In reality, these are not that many. Families are more likely to be wealthy enough to fall into a) or b), or be less well off and fall into d) and e)
d) Those stretching themselves to keep their children in their school, by reducing spending in the wider economy. They’ll be paying 20% on fees, but reduced spending elsewhere means less VAT, corporation tax, income tax, NIC etc. These other lost taxes haven’t been accounted for in the IFS figures.
e) Those who simply cannot afford VAT and who’ll move their children to state, or homeschool. State education comes at a cost of £8k per child, plus the lost VAT on fees. Home education might mean one parent gives up or reduces work, meaning they pay less income tax and NIC.