The Letters of Last Resort are letters written by the British prime minister to the British ballistic-missile submarine commanders (submarines armed with nuclear warheads.)
If you were the British Prime Minister, and an enemy nuclear attack has killed you and everyone in your administration and devastated the UK (you do not know who attacked, why, how, for what purpose, or the exact level of damage,) what would your Letters of Last Resort say?
I would have the commander put his submarine under allied command. If things are that desperate, the best hope left is to entrust the remnants of the UK force to intact, functioning governments. They can make the decision whether or not to nuke.
The meta-answer is: never let the world know there are such orders. That knowledge, alone, weakens them dangerously.
My vote is: transfer to allied command.
If you don’t know who attacked, who do you retaliate against?
Previous thread on the same topic you may find interesting 
Surrender and join the winners.
I agree with the join the allied command order. That is the only responsible thing I can think of. The UK only has four nuclear missile capable subs. They can cause a big headache to any nation but not completely destroy even small ones with that level of firepower. They would need to get aligned with the U.S. command very quickly because you can be rest assured that it wasn’t the U.S. that took out all of London and the rest of their own command.
I am not all that convinced that the simple existence of a no retaliation strike order is the straw that breaks the camel’s back. Fuck that, people aren’t going through that much trouble to start a war in this day and age simply because they think they can get away with it. The upheaval even to winners would be unheard of.
If I wrote that letter, I’d make it known that I’m no sociopath by publicizing the fact that I won’t retaliate. Its pointless, we’re just adding more misery to the world at that point. At no time will I ever believe that such a declaration will be the final thing convincing some tinpot despot to finally launch that attack he’s been gunning for. Either such a war was going to happen anyway or not at all. The orders make no difference
If the UK has its entire C&C wiped out without knowing who originated the attack, her Majesty’s forces are a joke.
Unless Sudan figures out how to fire Russian weapons, your tracking systems will know the oigin within minutes (if not seconds of launch).
If someone smuggles in enough weapons to detonate them in place, you really are screwed, but I’d go after Yemen, Pakistan, and maybe Iran. If they didn’t do it, they know who did, and were cheering on the sidelines.
I’d say get thee to an American port and put your sub at the US’ disposal. The Americans will know if there would be any point to using your weapons or not. To just fire them all out of revenge doesn’t seem right- millions of innocents would be killed and it might be that the war was already over.