Low to mid-end the selection is, predictably, limited. I'll second COS (have pieces in near mint condition that are well over ten years old). The Trend (pink label) and Premium (mostly online) and the Studio Collections from H&M. It's a bit hunting for the right fabrics but I have great things from Weekday and the odd gem from Monki. &Other Stories generally good too. Massimo Dutti isn't awful either, Uterque is of similar calibre but they both don't last quite like COS does at the same price point. I've stumbled across the odd piece in Muji.
Filippa K is a bit more mid-high but I have really great things from them too. Similar in price point, Comptoir des Cottoniers for coats and knitwear. I've bought the best ever Merino fine knit sweaters at Le Bon Marche (the high-end department store in Paris) from their own brand for very little money (30 or 40 euros) and they are virtually indestructible and withstand machine washing like no other (advertised as such), only available in their store though.
High end there's more choice though I am wary of things I bought over 10 years ago and if they are still made to the same quality as today, eg. Pringle since its takeover. I had a Gloverall coat as a teen and I'm pretty sure it would still be in great condition today had my father not accidentally dropped it behind the dryer, setting it and the dryer on fire . Case in point, the dryer came out with more damage than the coat, still unsalvageable though. I recently looked at a Gloverall coat again and it seems the fabric is now significantly thinner, it may be my teenage memory playing into it but it really weighed a tonne and the new ones don't.
Paul Smith is brilliant both in cut, fabric, and longevity. I have a coat that's lasted for forever and still looks brand new and for whatever reason, random retired couture seamstresses always strike up conversations with me about it because of how it's finished.
Max Mara and all its sister brands (Marella, Sportmax, Marina Rinaldi (Plus sized), Pennyblack, the only one where the quality dips somewhat is the entry level Max & Co but it's more due to fabric choices than manufacturing) are indestructible too IME and remain so to this day. I have several MM coats, one is at least over 15yo and all are still in great condition, lining still intact, no stray buttons, look brand new when returning from the dry cleansers again, etc. They also do the best pencil skirts as far as I'm concerned.
Lemaire does beautiful clothes that will last, and the collection he does for Uniqlo (the U collection) is a good budget alternative, the rest of Uniqlo has never impressed me much, to be honest.
Nicest Cashmere I have is from Prada, picked it up in an outlet for very little. Equally, Armani is very well cut and the quality remains throughout the years. The numerous outlet malls in Italy are definitely worth a detour IMO for these kinds of staple pieces.
Acne, indeed the boots and shoes in general last and last but the clothing is impeccably made too. Wood Wood, By Malene Birger, 2nd Day, and Hope Sthlm. also good. Have had nothing but good experience with Chloe shoes and bags. The See By Chloe diffusion line does great clothing but the shoes are less good IME. Carven all great. Salvatore Ferragamo for shoes and bags. It's not my style but Tod's and CarShoe are a bit underrated these days but quality wise they're excellent.
T by Alexander Wang has lasted me well too. Ditto Helmut Lang. Isabel Marant is one that isn't as good as it was, say 10-15yrs ago. I'd say the same for Marni, though at their core the quality is still good just not what it used to be. A.P.C. and Vanessa Seward also great long lasting pieces. Equipment silk blouses stay beautiful for a lifetime as far as I can tell. Recently discovered Soeur and Loreak Mendian in France and so far they're holding out well.
Some local brands I'm not sure that are as easily available in the UK, CLOSED, Humanoid, Frenken, Anecdote, and Vanilia for the Alpaca sweaters (on the high street level I much prefer Alpaca over Cashmere).
Hanro and Calida for cotton underwear. Schiesser for thermals. Although the sizing on those brands is bonkers, everything is giant sized, you need at least two sizes smaller. Rosemunde have the best vests ever.
Wolford and Fogal for tights. Kunert, Oroblu, and Falke are a good second.
There's probably more that I'm forgetting now. I do think that while it's all well and good to source these things like WhatsItAllAbout123 alluded with her high dry cleaning bill these things only last when you take care of them properly. It means washing things when things are actually dirty (especially wool doesn't get dirty quickly at all, airing them out often is more than enough, denim fanatics only ever stick jeans in the freezer, etc.), per manufacturer instruction with the right detergent (the dry clean only recommendations can be worked around if you're confident). Hardly any of my clothing goes into a dryer and mostly goes on a low spin cycle, everything is hung out on hangers and then usually steamed (if necessary) as that's more gentle on fabric and less hassle too. Wool gets washed by hand or a special cycle then placed on a towel where it's carefully rolled up to take out any excess moisture, then dried flat on a special rack. Most people don't want/have the inclination do go through all that and why washing only when dirty becomes a lot more sensible but that is how things keep their shape and material quality.
Whilst I'm here and if anyone is still bothered to read after that novel, local department store recently started selling Johnstons of Elgin Cashmere (Scottish I believe), any feedback on that as it's a new brand to me but appeared promising?