[Guest]

It is - however you will need to check the cover offered by the hire company and the cover offered by the excess company to make sure there is no "gap"

Often most cars that UK websites offer for hire in the US cover full insurance anyway so in most cases the excess cover is not required (this is not the case with Virgin Flydrives - where the basic cover is just short of worthless and often its cheaper to forget to pick up the included car and get a fully covered car from another provider than it is to upgrade the included cars insurance)