I also think a lot of schools (especially academies) are introducing new, increasingly silly and formal uniforms and rules around appearance which they seem to think will magically improve discipline. As if dressing up the kids like extras from Tom Brown's School Days will somehow vanish all the complex social and systemic problems that lead to poor behaviour. It's lazy and insulting, and the kids know it.
There have been quite a few studies looking into the effects of having a school uniform. They consistently find that teachers like uniforms but pupils hate them, which is unsurprising. The majority of studies also find that introducing uniform is linked to improved attendance, improved behaviour and improved academic results. The studies are not unanimous - a few found no effect and a tiny minority found an adverse effect. But the majority find a positive effect.
You are, of course, right that introducing a uniform will not make social and other problems disappear, nor will it stop poor behaviour completely. But, despite your scepticism, the evidence suggests that having a uniform does bring benefits.
workplace rules generally have some relevance to the practical function of the working environment
I have come across a lot of places where workplace rules on dress code and other subjects have little, if any, relevance to the practical function of the working environment. On the particular subject of uniform/dress code, an employer can enforce whatever they want within reason provided it is not discriminatory (or, if it is, the discriminatory aspects are a proportionate means of achieving a legitimate aim). So, for example, an IT consultancy a few years ago banned staff from having beards or moustaches purely because the MD didn't think they gave the right image. Similarly, an employee who was fired for wearing his hair longer than allowed by the employer's dress code lost his case for unfair dismissal.
You may not approve of such workplaces but there are many of them around.
The OP's child has repeatedly flouted the rules. If that happened in the workplace he could be sacked. If he doesn't want to be disciplined he should follow the rules. He can't pick and choose. If the OP doesn't think her son should be disciplined for his behaviour she needs to move him to a school with a more relaxed dress code.