Wasn't it something like a 3-4% reduction in private school pupil numbers? And some of those would have ended up homeschooled rather than state.

A rough calculation: £8,580 funding per child at state school, ~90,000 less in private schools and in state schools so about £770 million more required for state school funding and this measure is supposed to bring in about £1.7 billion a year...

So it looks like it would pay for itself?

Edit: We don't charge VAT on private healthcare - so charging it on private education looks a bit inconsistent to me.