The public school uniforms are heavily subsidised and students parents tend to buy them for them. If a family member cannot afford one, for whatever reason, they can have a confidential meeting with a yearmaster and the school will buy it for them (it’s the same uniform, the school just sends someone out to the uniform shop to buy a uniform of the correct size), but that rarely happens, as it is cheaper than buying the kid a shirt and shorts that aren’t a uniform. A lot of kids just live in their uniform weekdays, as they’re cheap hard-wearing clothes the parents don’t have to pay as much for.
No idea about private schools, but they’re probably richer families, I guess.