The Brexit transition period has ended and new rules on pet travel now apply.
From January 1, 2021 pet passports will no longer be valid. The EU has agreed that Great Britain should be given “part two listed” status, allowing pets to travel within its borders providing the owners to obtain an animal health certificate (AHC).
The AHC confirms that your pet is microchipped and vaccinated against rabies.
You will need to get a new certificate from your vet each time you travel with your pet and you must obtain it within 10 days of the date you travel.
The document is valid for four months: for a single trip into the EU, onward travel within the EU, and re-entry to Great Britain. All other requirements (microchip, rabies vaccine at least 21 days before traveling, tapeworm treatment) remained the same.
There will be no change to the current health preparations or documents for pets entering Britain from the EU or Northern Ireland from January 1, 2021.
More at source: https://www.gov.uk/guidance/pet-travel-to-and-from-great-britain