If you’re in Spain, Cyprus or Malta, you must display a GB sticker no matter what is on your number plate.
In EU Countries other than the 3 mentioned above you do not need a GB sticker if your number plate includes the GB identifier on its own or with the Union flag.
You do not need to display a GB sticker to drive in Ireland.
Apart from the 3 countries mention above you must display a GB sticker clearly on the rear of your vehicle if your number plate has any of the following:
A Euro symbol
A national flag of England, Scotland or Wales
Numbers and letters only – no flag or identifier
For more information, including documentation requirements here is a link to the UK Government advice for driving and using UK vehicles in the EUhttps://www.gov.uk/guidance/driving-in-the-eu#vehicle-registration-documents