They serve no purpose and promote conformity. Differences in income are expressed in other ways - shoes, bags, mobiles, in fact, even more so. Kids are judged by which school they go to (state/private). Thanks to the uniform you can tell by a mile off. Kids wear brands anyway - after school, at the weekends and in the holidays, so it's not any cheaper to wear a uniform to school. Consumerism is the most pronounced in the UK of all European countries.
Other countries like Sweden have no uniforms at school and few uniforms in work life, too, and rate much higher in terms of kids' happiness etc.
I'm not saying there's a causal link, but uniforms are not helping...