I'm not claiming smarts but my guess is it's purely a diplomatic gesture.

"Special Relationship"

https://en.wikipedia.org/wiki/Special_Relationship

IDK, but at this point its all pretty arbitrary however they do it since it seems to be spreading in some parts of the USA pretty well already. I guess less travel, whether it's EU, UK or even domestic, is better, but where from doesn't really matter as much anymore so Trump picked some for political purpose.

Because they dont inconvenience people with Temperature checks on arrival- even from Milan.

Because Boris is UK-Trump? (said only semi-seriously, I don't follow UK politics close enough to know for sure how good the comparison is)

I suspect it's a matter of the "special relationship" the US and UK have had for decades; Trump doesn't want to be seen as the one throwing our UK friends under the bus, especially with the promises he made during the lead-up to and implementation of Brexit.

They're not in the Schengen zone, so they can control who comes in?

Presumably it's an English speaking country with tighter ties.

Someone told Trump they left the EU so in his mind they aren't part of Europe anymore.