I believe that you have completely misunderstood the left wing attitude towards private schools. It is rooted in the belief that everyone should start off equal and progress on merit rather than get a head start in life based on where they went to school and who they met.
Which is laudable but flawed because of their lack of understanding of the education system they have. Choice of school is often poor.
As an example, my own children are girls. It has been proven without doubt that girls do better in single sex schools. They are more likely to study Stem subjects and grades are always higher.
There are no state girls schools anywhere near me, with the exception of Tiffin Girls which is selective based on academic excellence. Both of my children also need schools with small classes and good pastoral care for various medical reasons. Again - nothing is available.
Labour have this view that private or public schools are basically Eaton. The majority aren't. You find that probably around 60% to 70% of children at private school come from wealthy families. The remainder rely on bursaries, scholarships, remortgaging etc. A significant proportion of those children have additional needs such as autism, mental health issues such as anxiety or anorexia, ADD etc, or combinations of all of those. It's also common to get intake in Year 8 or at summer term of Year 7 after those children have been failed by the state schools.
I attended public school via an assisted place because I was both bright and because I could not cope with the state primary I was in. These days I would probably have an EHCP for Autism. In those days I was just refusing to go to school because it was incredibly boring and pointless. I could read fluently and do times tables when I started school. When I moved to Prep, the approach was completely different. The classes were smaller and I wasn't significantly ahead of my peers.
Until the state can provide smaller schools with smaller class sizes, the thing that removing assisted places, adding VAT etc does is to disenfranchise those children that are not wealthy. If your family is lucky to be in the millionaire category then price increases don't really mean anything. Thus Labour punishes the less well off and not the wealthy.
A more sensible approach would be to look to Finland. Increase the Education budget, reduce class sizes. Build more small schools and make it more appealing for small secondary schools to be viable. Actually learn from the private sector. If 30 pupils to a class worked well, all private schools would have 30 children to a class.