It was always pretty clear that this policy would not make money, and in fact might well cost money.
The LP has long had a faction that wants to make private education illegal, and the policy is mainly IMO about appealing to these people.
You can see it in the thread, they are just desperate to see people punished for having accessed private education.
But the fact that they don't seem to have considered increasing school provision to account for those leaving private education is nuts.