It's common in the UK to sell them both with a £ denominated value on them or the class of service. My brother bought loads of the latter a few years ago, which was a great idea, because as mentioned postal rates have increased significantly. The UK post office will also exchange your old unused stamps for new versions (now with QR codes or with sticky backs) for free. So even if they go out of date they alcan be upgraded at no cost.

USPS still sells denominated stamps from $0.01 to $10.00, in addition to "First Class Forever" stamps - see near the bottom of the filters sidebar on https://store.usps.com/store/stamps