With specific reference to Self Portrait clothes, which you mentioned, OP:
Their style, particularly the guipure lace dresses, has been copied endlessly on the high street but having seen the real thing in Harvey Nichols, and the copies, I’d buy the Self Portrait gear if you can afford it. The dresses just look better, the style is more refined and the colours are better. It would be obvious that the Self Portrait dress was more expensive than the ASOS copy.
I get the natural fibres thing, but some fabrics, like the lace used by Self Portrait, can’t easily be produced in cotton or silk.
I can buy posh clothes when I want, lucky me, but I choose carefully. It’s easy to pay £££ for something that you could easily replicate for much less, but some stuff just can’t be copied by cheaper operators.
I’m not in the Hermes and Chanel league, but I will buy the more accessible designers. I buy Vogue and Bazaar and some of the prices are barking.
Examples of stuff I’ve bought that was worth it:
a Roland Mouret navy cocktail dress I paid £1,000 for, ten years ago. It is stunning. The construction and material just couldn’t be replicated on the cheap.
a Missoni coatigan, £700 ten years ago. It is such a statement. The high street wouldn’t copy it because they don’t tend to deal in statements so much.
a Marella (a MaxMara brand) coat, bought six years ago for £400. Double faced wool, lovely, still looks like new and is beautifully cut.
a Hugo Boss dress, bought three years ago for £400. It moves beautifully, the construction is unusual, and I can’t imagine that the high street could ever copy it.
Jimmy Choo shoes: the only high street shop that even gets close to producing quality statement heels is Kurt Geiger, and they have been hit and miss in recent years. Karen Millen RIP used to get shoes spot on ten years ago but then they lost it.
For more workaday items that look posher than they are, I am a big fan of M&S and Reiss. When you get your eye in (as PPs note upthread) you can spot the good stuff. Winser London is more expensive, but do some real quality tailoring.