While Royal Mail phased out standard everyday non-barcoded stamps (the plain, single-colour stamps that only feature Queen Elizabeth II in profile) in 2023, they specifically exempted ALL Christmas and pictorial stamps (also known as commemorative and special stamps).
ALL CHRISTMAS AND PICTORIAL STAMPS REMAIN VALID, WITH OR WITHOUT A BARCODE.
Your items will go through the postal system as normal and will not incur any delays, surcharges or extra fees.

For further information, please visit the Royal Mail Website - Know where your old stamps are? │ Royal Mail Group Ltd