UK Hosts: Where do you buy bed sheets and duvet covers?

I see a lot of recommendations from US hosts to buy Costco linens for a good balance between value and quality/durability. But what about the UK? I was at a Costco and they sold a Kirkland set that seemed more luxury-tier, 680 thread count and £60 ($USD 80) for a set of duvet cover, fitted sheet and *4* (not 2) pillow cases. The extra pillow cases seem a waste.

I have tried a Dunelm sheet - quite solid but a little stiff and scratchy and about £20 for a double-size fitted sheet.

Any suggestions for value/quality sheets and duvet covers in the UK?