Copied from this RAC website. There's a bit more on there if you fancy a read...............
https://www.rac.co.uk/drive/travel/country/France/"Do I need a GB or UK sticker for driving in France?
From 28th September 2021, the distinguishing mark (or national identifier) displayed on vehicles registered in the United Kingdom that are driven abroad will change from GB to UK.
This means that vehicles registered in the UK must display the letters “UK” when driven in France.
The identifier can be incorporated in vehicle number plates (along with the Union Flag) or as a separate sticker. Note that vehicles featuring the letters GB together with the Council of Europe golden stars are no longer valid for driving abroad.
If your vehicle does not have the UK identifier within the number plate, you will require a UK sticker when driving in France. GB stickers will no longer be valid from the end of September."