There's nothing special about the US in
the UK list. There are only 10 countries in their green list.
Per a
CDC page, new cases in UK for the past 1 week are 39.7/100K (5.7/100K per day) while the US is 31.4/100K (4.5/100K per day).
Green: Australia: 0.3, Brunei: 0.6, Falkland: 0.0, Faroe: 0.0, Gibraltar: 0.0, Iceland: 0.0, Israel: 1.3, New Zealand: 0.2, Portugal: 39.2, Singapore: 3.5, South Georgia & South Sandwich Islands: -, St. Helena, Ascension, & Tristan de Cunha: 0.0.
Other than Portugal, all those are very few cases.
Vaccinations don't matter for their restrictions. Obviously they aren't going to say that "you're ok if you have two Moderna shots, but not if you had <some other vaccine from some other country>"
I just talked to someone in Taiwan who said outbound flights are full & tickets are $2K as people are leaving to go get vaccines elsewhere. A question for this person is whether he could fly back to Taiwan without a quarantine. (Btw, Taiwan (13.6) is on UK's amber list.)