"Note: Special Stamps with pictures on and Christmas Stamps without a barcode will continue to be valid and don't need to be swapped out."
https://www.royalmail.com/sending/barcoded-stamps#:~:text=old%20stamps%20are%3F-,You%20now%20have%20until%2031%20July%202023%20to%20use%20them,to%20all%20our%20regular%20stamps.
Happily, RM decided to give Christmas stamps the same perpetuity as commemorative issues.
"Originally you said Christmas stamps are being barcoded and non-barcoded Christmas stamps would become invalid as postage - has this changed?
Following our announcement on 1 Feb 2022, and subsequent discussions with key stakeholders, including feedback from our customers, we have removed Christmas stamps from our Swap Out scheme. These stamps are mostly kept by customers to send their Christmas cards and we anticipate that the vast majority will have been used by the end of this year. We are still adding a barcode to Christmas stamps, but non-barcoded Christmas stamps will remain valid so can continue to be used after 31 July 2023 and therefore there is no requirement to swap them out."