Thing is, as well as paying £20k to the school to educate our DC, we're already subbing the government an additional £22k (£15k extra income tax on the £35k additional income I need to earn to have £20k take home pay + £7k saving on not having to educate my DC). I give the government £22k more of what I earn than if I sent DC to state school. That's a pretty long way from being a subsidy! Obviously that goes up to me giving the government £26k when VAT is added.
It's worth it, because the education is absolutely brilliant - and is helping my DC to become the best version of herself she can be. Not unfair members-club privilege, just genuine personal growth and improvement.
But if the school have to cut their educational provision by 20%, giving her a worse educational experience with less opportunity to learn and grow, then it becomes less worthwhile. At a certain point of cutting back what they provide, it won't be any better than state + extra-curricular activities and tutoring. When that happens,I may as well cut my working days to part-time, take her to extra-curricular activities in my new free time, and enjoy not working so hard. Unfortunately, the state will be £22k per year worse off. But that's not my problem: I'm obviously going to do what works best for my family, same as everyone does.
That's why this policy is so poor: at 10% of kids switching to state (not necessarily immediately: it's more likely to be that it sways parents not to choose private over the next 7 years) then the state is actually worse off.