nooka there is a girl at DD's school with rainbow coloured dreadlocks - DD thinks she looks cool but has no desire to copy her - there are 2000 (yes, two thousand) children in her school and she is the only one with a dramatically non conventional hair style, (no uniform - Germany) though there are quite a few boys with long hair and a few girls with the side shaved and / or non-natural shades of red or pink hair.
If you don't make a massive fuss about what kids wear or what they do with their hair it appears most of them just pull on jeans and a T shirt and stick their hair in a ponytail... nothing to rebel against, no need to agonise about what to wear because you can always wear what your want - no big deal, to anyone.
The suits for 6th form idea is really bizarre if the justification is based on the world of work - how many jobs actually require a suit? How many of those 6th formers will be going straight into a suit wearing type of office job (rather than to uni where there is, I sincerely hope, not yet a dress code... or to work in IT, where the "uniform" is Geek T shirt and jeans...
Its over a decade since I left teaching in the UK, but hundreds of cumulative teaching hours certainly were lost to enforcing uniform rules back then - heads of department and senior managers especially would keep classes queued outside their rooms until everyone had their top buttons done up and their ties tied properly, generally letting them into the room a good 5 minutes per class after the lesson began - 30 minutes per teacher per day lost, before adding in the "serious" infringements which took up more time, and the year 10 and 11 bottom sets who always had members who would deliberately set out to derail classes with uniform infringements and ask teachers why they weren't "sorting out" whoever was violating the uniform code.
There were always kids in each year who would be sent in wearing dirty/ very outgrown/ torn/ faded uniform, so I am not sure it is a leveller at all. I think it is a spotlight and a focus, drawing attention to and focusing the minds of children and staff on clothing. If everyone is wearing the same, subtle differences stand out more not less. Children work harder to show their personality/ group identity through the way they subvert the uniform and through accessories/ shoes etc. and the rare non uniform days become a fashion parade because their rarity focusses the minds of the children on what is worn on those days.
I went to a private school and remember the entire school being made to stand on benches in the assembly hall so the head mistress could more easily inspect our socks...
and there was an entire mythology and slang vocabulary around the uniform...