Just to clarify this, but UK citizens are not barred from flying to the USA. The Presidential Proclamations actually states "The entry into the United States, as immigrants or nonimmigrants, of all aliens who were physically present within the United Kingdom, excluding overseas territories outside of Europe, or the Republic of Ireland during the 14-day period preceding their entry or attempted entry into the United States is hereby suspended and limited"

The prohibition is based not on citizenship but on physical presence within the UK. I am a UK citizen but would be permitted to travel to the US as I fall under one of the exceptions (i.e. I am a legal permanent resident / Green Card holder).

I would be able to fly back to the UK as there are currently no restrictions imposed by the UK Government on my arrival and then return to the US. I would be required to enter the USA via a designated airport and then self-quarantine on arrival but I am definitely not barred from travel.