We live abroad but the DCs attend a British school so have a really similar uniform to U.K. students. We have a cupboard stocked with M&S white shirts in every size for DS (10) and white blouses in every size for DD (just turned 7), bought in the sales when I've been in London visiting family. Ditto grey trousers for DS.
They have navy jumpers/cardigans which I sew the school logo onto but we can buy those in local shops.
The ludicrously expensive uniform items they need from the school shop are a tie for DS and a pinafore style skirt and knee socks for DD. DS has one tie and will have the same one from Y3 to Y6 - it's 3 years old now and looking OK (wet pressed nightly and machine washed weekly or when there is a visible stain). DD has one skirt which luckily is also machine washable. It's good quality, at least. However will have to be replaced when she finally grows out of it (there is still some hem to let down). The socks are a bit of a nightmare as they're ludicrously expensive and DDs feet seem to grow like a weed.
P.E. kit I also resent as it's branded and very expensive but terrible quality. We bought big and they will wear it for as long as possible!
Bags, coats, shoes, trainers, swimsuits, etc. (luckily we can buy these from anywhere) are replaced when necessary and we encourage the DCs to take good care of them, e.g. DS knows he must change into his playground trainers to play football at break rather than wear out and scuff his school shoes.
There's no way we could replace everything every August and it wouldn't really make sense as everything from June would still fit but then they'd have a growth spurt in October!
Luckily they are too little/unaware to demand new stationery for the start of term but I do pick up new packs of colouring pencils and pens and make sure their pencil cases are well stocked ready for the new school year. (They tend to get things like pencil cases in their stockings at Christmas...Santa is quite practical in our house
).