[user104658]

In an ideal world, the quality of education in mainstream schools would be high enough for private schools not to be at all necessary.

In reality, there are SOME wonderful state schools and some that are simply barely fit for purpose, and it's a postcode lottery, so all that would happen if private schools were outlawed, is that financially well of parents would move to the catchment areas of the best state schools. Which already happens to some extent. My eldest will be in high school in 2 years. Her primary school is excellent, but her catchment high school is awful. She's going for a private school scholarship but if not, we fully intend to relocate for high school.

So yeah to go back to the original point; I'd prefer it if every state school was up to scratch, but they're nowhere near, and the idea that anyone should "have to" send their kids off to a poor quality education even if they have the means 5o provide better, is quite horrifying.