UK and Northern Ireland VAT numbers
Since 1 January 2021, VIES has not validated UK (GB) numbers. It now covers businesses under the Northern Ireland Protocol, with the prefix XI. For GB numbers, GOV.UK has a service that shows whether a number is valid and the name and address registered to it.
The structure is nine digits, or twelve with a branch code, or five characters starting with GD or HA for government departments and health authorities. Pick GB or XI in the checker: the rule is the same, but the service that confirms it is different.