HOW will one of Barrow's Vanguard-class submarine crews respond if a nuclear weapon is fired at Britain?

Geographically, Britain is safer than a lot of countries, being an island on the edge of a block of allies.

If an intercontinental ballistic nuclear missile set off from Moscow or North Korea, it would take around 20 minutes to hit London.

En route, it would pass through the airspace of half a dozen Nato allies.

A text message is more likely, too, especially since the government has can easily contact every registered mobile phone the UK. What's more, it has the ability to restrict access to base transceiver stations (BTS), meaning mobile signal would only be available for the emergency services and energy companies.

Somewhere in the North Atlantic, one of Britain’s nuclear, Vanguard-class submarines is always patrolling, and this would be its moment.

Only the Prime Minister can authorise the deployment of its missiles.

From Pindar, a code would be sent by the PM via the Northwood Headquarters 100ft under the Chiltern Hills, through which all communications with the Vanguard-class submarines pass, and out to sea.

At every stage, the message is confirmed by two people, as a precaution against miscommunication or foul play.

The sub commander would then open two safes, one inside the other, and the trigger is then pulled.

In the worst scenario, there may be no nation left for them to defend. In that case, the same commander would go through a series of checks to assess the state of the nation.

One of these includes listening out for Radio 4’s Today programme for three days. Safe in the Worcestershire bunker, Radio 4 would be the only broadcaster left by this stage, utilising Wood Norton’s super high-frequency satellite; if Today fails to air for three days straight, it is safe for them to assume Britain has fallen.

And then, the submarine’s commander will read what’s known as ‘The Letter of Last Resort’, containing the Prime Minister’s final instructions.

Whenever a new PM is elected, the Cabinet Secretary immediately has them write four letters, one for each sub, with their recommendation for what to do if the chain of command has collapsed. Retaliate, don’t retaliate, place the boat under the control of the Australian or US Navy, or leave the decision up to the commander – those are the options.

Nobody knows what previous leaders have recommended; the letters are destroyed at the end of each premiership.