If you speak of existential war, then the US is just as threatened as the UK. Russian missiles know the way here as well as there. Matter of fact, imagine the first few lobbed are headed to the US before the UK sees any. Even with a conventional war, it's overly dramatic to talk as if the end is at hand, especially with the moat separating you from the bad guys. Saved you at least once in recent times, would work again.