I went to a non uniform primary. It'd be hell at high school.
I do remember one non uniform day in yr8 at high school. One girl - the class nerd - wore a completely yellow track suit. That was enough to convince me that the shitty uniform I hated wasn't so bad after all.
One of the issues is we assume that since other countries cope without uniform it would work in the UK.
The problem is that there's different cultural attitudes to fashion elsewhere. The UK is particularly faddy and particularly fast moving in terms of fashion. If you go to other European countries it's much less so and much more practically based. The UK leads fast fashion in terms of consumption. We are much more brand orientated than elsewhere too.
So applying this to High School in the UK wouldn't necessarily work, because out cultural differences would add to social pressures in this area rather than remove them.
I'd like to be wrong but I just think it's a much broader thing.
You can tell Brits a mile off from what they wear when you go abroad from the cut, colour, brand and styling.