My DC's primary school did not have a uniform, but it had a dress code; no t shirts with pictures or slogans, no jeans and no plimsolls (this was the 1970s). But due to parent pressure (not me) a uniform was introduced for those who wanted to wear it.
My children didn't because, as I said to the HM. As the children go through life they will need to understand that you need to dress appropriately for the job/occasion and I thought the best way to learn that was at school, where they were not constrained by a uniform but learnt that some items were acceptable wear at school and some weren't.
I think children in school uniform usually look scruffy, especially those at secondary, the reason is that as soon as you insist children wear a uniform they instantly start subverting it; the rolled up skirts the unbuttoned collar button and loose tie etc etc. I was a past master (mistress?) of the art. Carefully treading a line that could be guaranteed to leave my HM, angry but frustrated, because I knew just how far to go without, quite, breaking the rules.