[eroded]

> one armed nuclear sub could be wiped out fairly easily if we're talking about a power like this, right?

The one that's in dock for service and repair, yes. But the others could be at sea anywhere in the world, in an unknown location.

> Would all four, if armed, be able to do much damage back?

Each sub can carry 16 x Trident D5 ballistic missiles, with a range of 6,000 mi. Each missile carries multiple warheads, each with much higher yields than those used at Hiroshima and Nagasaki. So certainly the potential to do lots of damage.

The US has a similar setup with their Ohio-class submarines, though I don't believe they carry such letters or equivalent.

[ams6110]

I would find it hard to believe that USN submarines do not have equivalent "standing orders" on what to do if after an attack the command hierarchy is unavailable.

Whether these are in the form of sealed handwritten letters from the President, I don't know.

[mikeash]

To elaborate: each submarine carries 48 warheads (they can carry much more, but don't). I can't find much info on the warheads with a cursory search, but Wikipedia suggests a yield of 100kt, which is entirely plausible. The missile is able to put the warhead into a circle with a radius of about 100 meters.

For a "last resort" use like this, counterforce (i.e. attacking the enemy's military) is probably pointless. I imagine you'd go for cities if you wanted to cause as much hurt as possible.

A single 100kt warhead detonated over, for example, Manhattan would devastate Manhattan and significantly damage surrounding areas. Deaths would probably be in the millions. Add in a few more scattered around intelligently and that's the entire metro region wrecked. If we figure four is enough to do NYC in decently, that means one of these submarines could wreck twelve cities that way. More (up to 48) if you just wanted to smash the core areas. Or you can mix and match.

If it was targeted at the US (don't ask me why), imagine DC, NYC, LA, Chicago, all completely flattened, along with a bunch more. Most of the population of the country as a whole would survive, but the country itself would be pretty thoroughly wrecked.

[gordonguthrie]

The UK 'independent' nuclear deterrent isn't. Unlike the French one, it uses US missiles whose targets and strike pattern is set if North Carolina.

[gaius]

Only in the same way that Honda loads your GPS at the factory in Japan, but you can drive it wherever you like.

[tobylane]

Can you reword that? I didn't understand it.

[gordonguthrie]

The US sets the targets for UK cruise missiles. The French set the targets for French cruise missiles. The Frogs could bomb NY, the Brits, not so much...

[mikeash]

Nitpick: the British submarines carry ICBMs (technically, SLBMs, which is mostly just an ICBM launched from a submarine), not cruise missiles.

(In case anyone is wondering what the difference is, a cruise missile is basically a suicidal unmanned drone that flies like an airplane, while an ICBM is a rocket that goes into space and then falls back down on its target after burning its fuel.)

[arethuza]

The old UK V-bomber fleet was definitely targeted at Soviet cities and any air defense sites that would stop them getting there.