Public service announcement: did you know that all new stamps are now barcoded, and non-barcoded stamps won't be valid after 31st January 2023? This had passed me by, but fortunately the eagle eyes of Mr Gasp0de had spotted it. You can get old stamps replaced with barcoded stamps at no cost to you except an envelope. I've just sent ours off.
We had quite a few second class stamps hanging around. We rarely post anything any more, so it could be years since I bought them. Fortunately, Royal Mail don't want to know how much I originally paid, and will just give us new 2nd class stamps which will probably see us out.
All details here: www.royalmail.com/sending/barcoded-stamps#paragraph-11255 There's a link on that page to a 2 page PDF which you have to print off and fill in. If you don't have a printer you can ask for a form to be sent to you. You then send form and stamps back to Royal Mail in an envelope, but it's Freepost, so, ironically, no stamp needed to get it there.
It does say on the form, but the current price for a 1st class stamp is 95p and 2nd class is 68p. 