That is odd about the UK vs JP website, I tend to look on Net A Porter or similar sites as they usually have the most pictures and often video too.
I agree laundry habits might come into it. I'm always confused about people saying things fall apart or lose shape quickly after purchase, maybe it is just about how discerning I am when I'm buying but I think it's probably much more down to washing and drying (I always dry knitwear flat and avoid washing it as much as possible) and the occasional bad batch. I'm not convinced avoiding low priced items automatically mean you're less likely to encounter it, you're just somewhat reducing the odds. The only issues I've ever encountered quickly after put, and that's at all price points is stitching or buttons coming lose.
I've not noticed a decline at Cos but I think I last bought a dress there in lockdown one and only visited the shop occasionally since.
Uniqlo U, it depends on the person, materials have changed from previous collections so the quality or fit of it isn't what it used to. For me it's a combination of boxy fits and that the size (xxs) I need is based on an Asian average height (it's their size xs) and that is a couple of cms shorter than me so it fits weird but the boxiness means a size up that utilises the western average height feels much too big.