This is a poorly thought out policy!
In an ideal world there would be sufficient state school places for children of school age, state schools would be adequately funded and all provide a good level of education enabling each child to achieve the best of their ability.
The reality is there are insufficient places, schools have been woefully underfunded for over a decade and the emphasis is all about meeting the band of expectation - no incentive whatsoever to exceed expectation.
Children attending independent schools are not all from wealthy families. A large proportion are from fairly normal earning families but who work additional hours/ jobs and go without luxuries to be able to pay the fees. There are also children who are on scholarships and bursaries.
The genuinely wealthy families will simply pay the extra fees and life continues as normal. These people are a minority.
The families that go without luxuries to pay will in all likelihood be priced out, scholarships and bursaries will be a thing of the past with the loss of charitable status.
State schools have not been promised extra funding and there is already a shortage of places. How will extra places be created other than increasing class sizes? Let's also not forget the parents paying for independent education are still financially contributing to the state system.
If the necessary investment was made into the state system and every child had the opportunity to attend a good school with good facilities and opportunity to achieve the best of their ability then there would be no need for independent schools - other than for the super wealthy who will follow that route regardless. Only at that stage woukd the argument to tax the school fees make economic sense. To do so without the state system being overhauled first would remove the little social mobility that remains.
My three children all went to the local state school, I was morally against independent schools. Through various circumstances I witnessed first hand how easy it is for the state system to fail certain children - and the ridiculous strain it is under. We moved all three of our children to an independent school as a result and have never regretted doing so - although the financial strain was crippling.
It wasn't a luxury, it was necessity for each of them to have the opportunity to achieve the best of their ability.
The elder two are now back in the state system - they both were fortunate to be offered places at fantastic state secondaries. My youngest has a few years left at primary, whether she remains in independent school very much depends on which state school she is offered for secondary.
The policy should be raising the bar and the provision of a first class education available for all - only when that has been achieved should they look to impose tax on school fees (ie when choosing to educate independently genuinely is a luxury).