The UK is an Island nation with entry controls - passports are required even from the EU and there is at least token screening at the airports and docks of people with fever or other virus symptoms.

Not that it’s the same thing, but being an island (so no walking traffic) and having some screening in place means that the UK simply doesn’t have certain diseases (like rabies) that are at least somewhat common in Europe.

Honestly though, the UK is one of, if not THE, staunchest ally of the US and banning its citizens would cause some minor political issues.