Not 100% sure, but I remember reading that for nice soft sheets look for a minimum of a 300 thread count.
I know most people love White Company sheets, and they do high thread counts, it depends on how much you want to budget.
I like dark coloured sheets and am waiting to pick up some Gaspa sheets from Ikea which are a 310 thread count, feel nice and soft and I think they're a good price. They say in each listing what thread count they are and there seems to be a nice variety of styles.
House of Fraser also do nice high thread count sheets and they have a sale on at the moment.
Hmm, thank you. I'm annoyed that I can't remember what mine are - the light ones are lovely but the thicker ones are a bit too heavy. Nice for winter though. I forgot about White Company, I'll have a look. I have Debenhams vouchers but their selection is rubbish.
Balancing cost against quality you want 300-400 count. You can get these quite readily in TKMaxx without spending a fortune. I've just bought nice Fenn Wright Manson 300 count sheets from there which are lovely.