As for the rules around that, they can be very limited. I see that Camden School, for instance, simply says that "Students at the school may wear what they wish, but are expected to dress appropriately for an educational environment. Students should not dress in a manner likely to offend public decency, or wear clothing which prevents or interferes with identification through recognition of the whole face for security and safeguarding purposes or communication and learning." Sounds eminently sensible to me.
It sounds sensible to me too.
The problem is there are sadly people who aren't sensible and lack common sense.
These are the people who see "no trainers" in the uniform, buy trainers, then get annoyed and go to the local press when the school says trainers aren't shoes.
Or the school uniform says natural makeup and no nail varnish, their child goes in with a full orange fake tan, full face of makeup, highlighter like a disco ball, fake nails, and then complains when school sanction for uniform infringement.
Many school rules end up having to be made to cater for either the idiots of the world or the people who seem to think even the most basic rules don't apply to the..
So whilst you and I would probably see that reasonable school policy and dress our DC accordingly, I guarantee you that others would send their children to school in inappropriate attire and then spend hours up in school arguing "it says appropriate... It doesn't say you can't wear..." because there are people who do this over a child wearing thin leggings instead of school trousers because 'technically they are treggings and your policy only says no leggings".