If you can find them, Lidl ones are a good choice, or Coleman, which are identical, and as I’ve got both, I can attest to that fact. The Lidl one was about £20, the Coleman bag about £40; the reason I have both is I went on holiday and discovered I’d left my bag at home 130-odd miles away, so found an outdoor equipment store on the way to Kingsbridge and bought another. Great bags for the money, a cotton bag liner and a decent self-inflating mat and you’re sorted. Ten days camping with my setup was really comfy and nice and snug at night.
Of course, you could just get a self-inflating mat and take a large duvet and sleep in the bag liner, either would work.