DH and I tend to use M&S for good quality neutral basics. I really like the fact that it's a British company and its ethical standards are high. However, I'm not in the habit of looking there for distinctive pieces, because of the way the shop is set out and the fact that much of the clothing is too fussy for my taste or is in synthetic fabrics.
We buy:
men's trousers
skinny, extra long school trousers for DS
socks
jeggings for me
underwear (but not the bras- not a great fit IMO)
tights
shoes for work as they do wide fit.
Clothes for DS2 (13), who is not concerned with fashion, but will wear only ethically-produced clothes.
I tend to buy "higher end" tops - Scottish cashmere jumpers, and silk shirts.
This thread has made me think, though, that if I can be bothered to navigate the confusing layout, there are some plainer, good quality, things I'd like.