ASDA and Tesco are founding members of the Ethical Trading Initiative who, alongside Sainsburys and M&S so their ethical standards are pretty easy to find. I'm not saying there aren't likely issues in their supply chains, but the idea that supermarket clothes are automatically worse ethically than others doesn't hold water.
Why are people surprised at the cost of school uniform?
They vary so widely in cost and the rules change pretty frequently, so it's really no wonder some parents are surprised. I'm often surprised by how much others have to spend on uniforms.
Having not grown up in the UK (we had dress codes, but no uniforms) and home educated my kids for years before my Y7 DD made the choice to transfer to school mid-year, it was a bit of an unexpected bump in the budget when she got a place to have a couple weeks that cost us about £200 for all the uniform parts. They were kind to offer her a few weeks without the pricey blazer which I'm glad we didn't have to take as the transition was rocky as it is and her school is fairly inexpensive compared to many of the prices I see on here regularly, but it was a surprise. Most might not need to do it from scratch as we did, but having to move into a new area or having a new head can bring a lot of unexpected costs.
We had a secondary school near me that went through 4 headteachers in 3 years. Every single head brought in changes to the uniform policy. Every time there were complaints in the paper and across various social media - and I really can't blame them the last couple times. It was ridiculous and obviously didn't do anything to keep having headteachers 'make their mark' or whatever by doing that and making parents pay for it.
My DD's school is more flexible than many other secondary schools around here, but even they had rule change just after she started so skirts are now only meant to be bought at the uniform supplier shop, alongside the previous requirements that the blazer and PE shirt had to be bought there (everything else can be bought anywhere, alongside the PE shorts if choosing shorts over tracksuit bottoms in the right colour).
Also, alongside other schools uniform changes, there have been 3 new schools in my area in the last 5 years, including the one my DD attends. I'd have loved that as an option, but it's far less of an option for many than it seems to be expected with how things are going these days.