It varies depending on where in the US you're going, and what time of year it is.
Normally the Eastern time zone is 5 hours earlier than the UK, so noon in London is 7 a.m. in New York.
Central time zone is 6 hours earlier than the UK, so noon in London is 6 a.m. in Chicago or Dallas.
Mountain time zone is 7 hours earlier than the UK, so noon in London is 5 a.m. in Denver
Pacific time zone is 8 hours earlier than the UK, so noon in London is 4 a.m. in Los Angeles.
Alaska time zone is 9 hours earlier than the UK, so noon in London is 3 a.m. in Anchorage.
Hawaii-Aleutian time zone is 10 hours earlier than the UK, so noon in London is 2 a.m. in Honolulu, except when London is on Summer Time, as Hawaii does not use daylight saving/summer time, so then it's 1 a.m. in Honolulu (though the Western Aleutian islands in Alaska do use DST).
However, between the second Sunday in March and the last Sunday in March, and between the last Sunday in October and the first Sunday in November the difference is generally one hour less, because North America starts Daylight Saving Time earlier, and ends it later, than Europe does Summer Time.
There are a few further complications, in that apart from Hawaii, Arizona (in the Mountain time zone) also does not observe DST, but the Navajo Nation, in north eastern Arizona, does observe DST; but the Hopi Nation, which is entirely surrounded by the Navajo Nation, does not observe DST.