Most of my (day to day) clothing is from Mountain Warehouse, Go Outdoors and Sports Direct, plus a few items from Sainsbury's/Next/M&S. I am very "unfashionable" - I don't like jeans, or any branded items. Plain is the way to go!
Stock clothing for me is cargo trousers, plain T shirt. Jumper/fleece of varying warmth depending on the temperature (most of these are university society branded items). Almost always my trusty black waterproof(ish) coat on top, unless it's the middle of summer - I've had this coat for probably approaching a decade and it's still fine, apart from the velcro bits no longer being sticky. Was fairly cheap, no more than £50 at the time. Shoes either robust trainers (with half the sole on one shoe no longer being fully attached!) or "walking trainers".
Going up a level of formality, trousers get swapped for either black/navy "semi-smart" items (there's probably a name) or smart black, and a smart shirt (white or pale blue; assorted sources, including charity shops). When required, a plain navy V neck jumper adds warmth. Smart black shoes (from Clarks, had them at school and they're still fine, although admittedly I don't wear them too often). I also have some smart black shirts for "all black" occasions, eg: concerts.
At the top level, I have a black suit and a blue suit (Slater's), plus a grey jacket (Next, massive discount). Components can be mixed and matched if desired.
I also have a couple of proper outdoor clothing for hiking, mainly some reasonable outdoor trousers (Go Outdoors) and a few "performance" shirts (Mountain Warehouse) and fleece (Mountain Equipment), plus walking boots (Berghaus)*. I've also recently bought a very waterproof coat (no more than £100) and a few thermal items (all Mountain Warehouse; their student discount is quite handy!), but generally my normal day to day clothing is fine for outdoor activities (unless the weather is poor).
I would like, at some point, to get a full Peaky Blinders-style clothing set, just because. But other than that, I have no particular desires; items are replaced occasionally if they get too ragged, but that's mainly it.
*Fun story - I did a weekend of hiking in the Peak District before Christmas. In £26 Sports Direct steel toes...
(I may have picked up the wrong bag of shoes on the way out of the house
My "task" shoes live in bags, usually because they're muddy)