I went into the Sézanne shop in Paris, very excited, but found their quality not that great: about right for the prices, but not as good value as I'd been led to believe.
I try to buy things that are good quality for the price - you rarely get good quality, that lasts when you pay a low price, but you don't always get it when you pay a higher price. So I look for really good quality, and pay accordingly.
Besides the already-mentioned Max Mara, Margaret Howell, & Sandro, I also like Hugo Boss, Agnès B, & Paul Smith, and I think good online and physical shops are:
www.madham.co.uk/latest.php
(Womenswear for female barristers, so lots of beautiful black suits and dresses, but much more besides)
www.morganclare.co.uk/shop/
(Shop in Harrogate, good online)
For knitwear/t shirts I like Sunspel, John Smedley, and special cashmere from Aethel
www.aethel.com
and Begg
www.beggxco.com