'Upon taking office, the prime minister writes four identical letters of last resort, each of which is locked in a safe on board the Vanguard submarines. If contact with the UK is lost, the commanding officer of a submarine has to follow the instructions in the letter if they believe that the United Kingdom has suffered an overwhelming attack. Options include retaliating with nuclear weapons, not retaliating, and putting the submarine under the command of an ally. The exact content of the letters is never disclosed, and they are destroyed without being read upon the election of a new prime minister.
Imagine if for some freakish coincidence the letter said to put the subs under the command of the US. Would the attack make them not an ally any more and thus void? What would they do in that situation?