At best this policy will directly net the treasury half of that, or to put it in context if every penny directly raised from this policy went to education it would increase the education budget by 0.4%, indirectly (through increased spending power of parents) there may be more VAT raised but that is both hard to quantify and it certainly won’t end up being ring fenced for education.
If these privately educated children end up in better jobs with better pay than their state educated equivalents (an argument made in this thread) then by reducing attendance at private school is harmful to the future tax receipts if this country. We can’t have it both ways.
I’d conclude by saying education is not goods it’s an investment and should not be subject to VAT, within the same investment context we should be spending much more on state provision on education, it should be up by at least 30% from current levels. Education is one of those things that should more than pay for itself in the long term so spending more on it is a bit of a no brainer.