You jammy bugger with that Brora cardigan! Anyway good idea, op:
The M&S St Micheal label - I'm sure it'll be collectible one day.
Aquascutum - fantastic quality. I think their raincoats were better than Burberry.
Dannimac - likewise, mainly the raincoats.
Jaeger - the old school brand not the M&S version. If you're not sure which it is, pick it up and go by the weight.
Windsmoor, Easter, Country Casuals, Planet - yes, the styles will look very dated but I've picked up a couple of things purely for the fabric. Admittedly I've not done anything with them yet.
Top Shop and Warehouse from the early noughties. So much better quality compared to now.
Regardless of label:
Silk shirts.
Anything linen if there's enough fabric and it's heavy enough, especially Irish linen. I check for old linen tablecloths too.
Anything in Harris or Donegal tweed.
Menswear:
John Smedley knitwear - they do women's stuff too but I find men's more often.
Horne Brothers - I found a fantastic men's cardigan, heavier than a lot of modern coats.
William Hunt knitwear.
I'll see what else I can think of.