I tend to get my best things from designers that specialise in doing just one thing - eg just knitwear, just jeans.
Lots of good British designers for knitwear: Oubas, &Daughter, Genevieve Sweeney. European brands like Moss & Cable, Ami Amalia, Knotty Ones, Babaa, L’Envers.
Jeans from Huit, Blackhorse Lane Ateliers, Donna Aida as well as the bigger brands like 7 For All Mankind, J Brand, Slvrlake (the latter ones I get from the Outnet).
Coats I like to buy vintage wool/cashmere (my favourite ones are from Max Mara and 80s Jaegar) but love brands like Stand Studio for faux fur/aviator jackets. Puffers I’ve got from Elvine, Embassy of Bricks and Logs.
Shoes and boots - Grenson, DMs, Solovair, Bhava.
All of the above have been great in terms of quality.
Also love finding little boutiques as they’ve done a lot of the hard work in finding good brands for you - Biscuit, 32 The Guild, Lorna Ruby, Meet Bernard are all good physical stores with online shops.