Having received an update from the UK Government, from 28 September 2021, you need a UK sticker instead of a GB sticker.
If your number plate includes the UK identifier with the Union flag (also known as the Union Jack), you do not need a UK sticker.
However, you will need to display a UK sticker clearly on the rear of your vehicle if your number plate has any of the following:
· a GB identifier with the Union flag
· a Euro symbol
· a national flag of England, Scotland or Wales
· numbers and letters only – no flag or identifier
If you’re in Spain, Cyprus or Malta, you must display a UK sticker no matter what is on your number plate.
You do not need a UK sticker or number plate to drive in Ireland.
https://www.gov.uk/guidance/driving-in-the-eu