Can I still use old 1st and 2nd class stamps?

The short answer is No, but royal mail as of the time of writing (April 2024) are still allowing customers to swap out the old red and blue stamps. You can do this by visiting https://www.royalmail.com/sending/barcoded-stamps 

If you are in need of some new stamps you can buy them here we are an official royal mail retailer.