One of the first tasks for a newly elected British Prime Minister is to write four letters to the commanding officers of the four British Vanguard-class submarines. These letters give orders of what to do in the event of the Prime Ministers death if an enemy nuclear strike has destroyed the British government.

The letters are stored in a safe, within a safe, within the submarines and are destroyed unopened after the Prime Minister leaves office.

What would you write in your Letter of Last Resort?

Dear Captain.

You are reading this because the British government has been destroyed and unfortunately I, the PM, is dead.

Hopefully you and the other submarines are still active. It is very important that you find safety in the hands of other Commonwealth nations, ideally Canada or Australia.

If this is not possible then you are to give yourselves up to a long standing NATO ally, ideally the USA or France.

If no ally is remaining then retaliation will be pointless, but do what you want.

Good lick chaps.
PM

Something like that but with more specific details.