The UK moved to stamps with unique QR codes back in 2023 for the bulk of its stock.

https://www.royalmail.com/sending/barcoded-stamps

There are still some "Forever" stamps (those with pictures on or Christmas Stamps) but the old non-QR code non-forever stamps stopped being legal a while back (but you can always swap them for the new ones for free).

I've had people try and reuse a QR-coded stamp, you just get a postcard saying there's an item of mail with invalid postage for you to collect (and pay for). You get to look at it before you decide whether to pay the fine or not and then your argument is with the sender to reimburse you or not.

I thought "forever" referenced the Royal Mail's typical transit time.