There is also a standing order that if the UK government is decapitated, its nuclear submarines are to take orders from the US president. (The actual orders ar

There is also a standing order that if the UK government is decapitated, its nuclear submarines are to take orders from the US president. (The actual orders are of course secret and sealed until that time comes, and could be AU/NZ/CA depending on the persuasion of that particular prime minister. However the US is the most logical choice as the other countries have no nukes themselves and would likely be looking to the US for guidance anyway.)