I was JUST thinking about the UK brand Poetry before I saw this thread, so I will use them as my example/contribution. Agree with so much that has already been said above.
I am attracted to neutral tones and simple designs, especially in linen, cotton or natural wools. Poetry always catches my attention, but I have never purchased from them new, because their marketing and reviews put me off.
Most of the online reviews, and I am assuming most of them are left by middle aged and older women (I am 49), describe the clothing as dramatically oversized - and not in a good way. At 5'4 and on the slimmer, small boned side I do not believe many brands aimed at the over 40's consider smaller framed women (looking at you MH, Toast, etc!).
No brand can represent all of us, whether that's bodytype, height or tastes. But there is a persistent association of higher quality, naturally sourced garments with taller, broader physiques. I manage, of course, since the market in general caters dominantly for slender women, but it is awkward when the styles and fit at Mango, Topshop and Asos are more suitable for your bodyshape than Margaret Howell. It depresses me as I don't want to use those brands. I Love MH!
So who is Poetry's target woman?
She has some money to throw around, and she isn't afraid of the sun. She is probably over 50, and likes a bit of drape. She is likely retired, slightly creative and not terribly mindful of cutting edge trends. She is also very tall and strident, tanned and well travelled.
The models are very tall, slim to curvy, rarely fair skinned or blonde. I see a glowing olive to dark skin bias in most of their campaigns, because the colours and settings compliment them. There are no short, youthful, pale redheads or lighter blondes like you might see modelling clothes at Asos. The Poetry woman is mature, amazonian, understated.
MH, COS, Toast, etc are similar with regards to height and build, although a touch on the slimmer side and less fixated with glowing tans and mediterranean cruise connotations. They might skew a bit younger, especially the models (as the OP mentions above), but I too rarely see women under 40 wearing these brands.
These women are 'makers', mothers, environmentally aware, expressive and mindful of 'protecting' (or projecting?) their identity against a sea of synthetic clones and Hollywood inspired gloss.
They have money to spend and lean towards the arts, which might entail displaying their curated collections of beach stones and seaweed on instagram.
The COS, MH woman might be more urban, city professional; she prefers a bit of structure and can lean towards monochrome (you rarely spy black as a colour option at Poetry, Toast, etc). This is a more sparse aesthetic, but highly considered.