If you really want to know, higher thread count generally means nicer, smoother sheets that feel less scratchy and softer. 300 is where the semi decent quality starts but is where most of the cheaper stuff aims for and a lot of it isn't great quality. 500 is where the better brands tend to aim their midrange good stuff at and a lot of 500 thread count stuff is really great. when you go above that you get two types of thing. You can get "real" high thread count stuff, anything from 700-2000 with rapidly increasing prices. But you can also get "fake" or cheap high thread count stuff where they twist several threads together to up the thread count on the cheap. Because the cost comes in having incredibly tightly packed threads flat next to each other. Taking a bunch separately, twisting it and then using that to make an effectively much lower thread count sheet where every thread counts as 3-5 instead.... you're basically buying a lower thread count sheet that won't feel anywhere near as smooth or nice. I'd be cautious of any you know £30-50 1000 thread count sheet as being the cheap type that won't actually be that great. Stick with something 500 thread count and you likely won't go very wrong... quality 500 thread count stuff is WAY nicer than basic bedding, and way higher thread count doesn't make an awful lot of difference.
I've got some 300 thread count stuff from M&S that was fairly cheap, and some 500 or 600 thread count stuff from Debenhams that is very very noticeably softer/smoother/better quality. It was like £20 fitted sheet from M&S and would have been £40-50 for a flat sheet from Debenhams but they were all in the sale and closer to 15-20 a sheet.
Also worth noting that dust mite covers tend to be 300-500 thread count because the threads get close and tight enough that the dust mites can't get through easily. So higher thread count sheets should help keep dust mites at bay.
IIRC the Debenhams stuff was called Sheridan or something similar to that. I never got the duvet cover because even in the sale it was something like £70. As what a lot of americans/hotels do, a much cheaper £20 flat sheet, then have the duvet and whatever duvet cover you want above that.