Gap Modern. I've bought 3 crew necks and they're perfect; soft, just the right thickness, longish, cotton with a bit of stretch and they're reduced at the moment.
I haven't washed them yet but so far I'm really impressed.
https://www.gap.co.uk/gap/modern-crewneck-t-shirt/000241902.html?dwvar000241902color=000241902004&dwvar000241902fit=regular&cgid=#q=Modern%2Btee&lang=en_GB&start=11