Just seen this on Money Saving Expert:
Royal Mail is shaking up the rules on stamps, so if you've got a stash that say '1st' or '2nd' class on them then by 31 January 2023 you'll no longer be able to simply stick 'em on a letter and use them. This is because Royal Mail is moving to a new barcode system, which these stamps don't have.
If you try to use these stamps – which impacts all those with the Queen's head on them that say '1st' or '2nd' class, or any other price, or they're Christmas-themed – after this cut-off date they will not be valid and you will have to pay a surcharge. Though Royal Mail is yet to confirm what this surcharge is or how it will work in practice.
"Special" commemorative stamps, such as the Doctor Who collection, are the only exception, as these are not being phased out and can continue to be used after 31 January 2023.
Royal Mail will let you 'swap out' these stamps for free
You will, however, be able to exchange current stamps for barcoded ones for free under Royal Mail's new 'Swap Out' scheme, which opens on 31 March 2022 and will run until 31 March 2023.
To do this, you just need to fill in a 'Swap Out' form from its website (the page isn't live yet), or call it, or get one from a local delivery office (not a post office though). You'll then have to post back the stamps you want to swap to a Freepost address. We're waiting to find out if there are any limits on the numbers you can swap and if you're covered for non-delivery.
www.moneysavingexpert.com/news/2022/02/anyone-with-stacks-of-stamps-at-home-has-less-than-one-year-to-u/