I suspect UK was left off because POTUS has resorts in Ireland and Scotland, and no where in Europe. There's nothing credible about UK being off the list while every surrounding country is on it.
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.
Note that technically the UK does not require passports for entry from the EU - identity cards are also fine.
Having travelled back to the UK from a Schengen country in the past few days, I can also confirm that there wasn’t so much as a poster in the airport, let alone any actual screening.
I don't think I've ever had anything that even counts as token screening flying into the UK from Europe - passport check is automated and I've never even seen someone at customs for many years.
No offence intended to Americans, but those guys are culturally much tougher negotiators than us. When they know they have leverage over you they don't hold back at all. We get caught out by this over and over again because we speak the same language so we think we understand what's going on, but we aren't awake to the nuances.
You might want to google Tizard Exchange for what happens when the US has us over a barrel. Or look at the outcomes of pretty much any attempt to extradite an American citizen vs US attempts to extradite a UK citizen.
