There are 2 issues here:
The first is that acceptable uniform is nearly always outlined on the school website, or at the open evening, or an a leaflet etc. Every generation has tried to bend the rules so they can look a bit more fashionable, but this is exactly what the schools are trying to prevent i.e. kids getting picked on becasue they're not wearing the coolest type of black trousers or shoes. The clue is in the name "uniform" so everyone wears the same so no-one is regarded as "in" or "out" of what's deemed fashionable or trendy.
The second issue is one of approved suppliers. This can be a bit of a con but as long as they've outlined where they need to buy stuff from, there shouldn't be any surprises or issues. We had to buy the entire kit (minus white shirt, which can be generic) and it cost £100. Provisions were put in place for those on lower incomes (second hand shop, and other options were offered).
I hasten to add some of these parents might have bought non-compliant uniform in good faith. E.g. at my son's school there is guidance for shoes, with pictures of 5 examples of acceptable shoes and 5 of not acceptable shoes. There may well be grey areas so some might have fallen foul of that, but at the end of the day it's about common sense and if you make a mistake you have to suck it up and buy compliant footwear.
My son's school is very clear on what happens if you're in the wrong uniform - you have to go to the office and obtain spare uniform and are expected to hand it back whenever you've bought the correct stuff (in a resonable time frame). I don't see much issue in what was quoted in the Guardian:
"They were given a grace period but, when that ended, the school is alleged to have taken further action - angering parents. It is claimed some pupils who continued to wear the "stretch" trousers were put in isolation, meaning they missed lessons."
That is exactly how things operate in the forces, at work, and at school. Preparing kids for adulthood is a fundamental part of school. So not really an issue I can give much sympathy to.