Re: .UK or .GB??
Vehicle registration plates were first used back in 1903
GB = Great Britain
UK = United Kingdom of Great Britain and Northern Ireland
GB and IRL for Ireland were the original designations.
Expanding GB to cover Northern Ireland has been argued over since Irish independence. It's still a sensitive topic.
Car racing was originally banned in GB but permitted in IRL. The first car races were therefore held in Ireland, which is why British racing cars were green.