If you are limiting this exercise to linking mainland USA (i.e. ignoring islands along the coastline) to mainland UK (which would include Northern Ireland and sort of contradicts the first proviso) then the two closest points (Belleek, Fermanagh and Hamlin, Maine - neither of which are actually on a coast) are around 2,580 miles apart.
Thus your journey would take a shade under 37 hours non-stop.
Coastline to coastline, such as Scottish Highlands to Bay of Fundy, is around 2,680 miles.