Hacking UK Trident: A Growing Threat
basicint.org
basicint.org
You are correct if you're talking about Britain: Royal Navy SSBN captains are able to launch missiles independent of government orders and there are no PALs involved in the UK nuclear deterrent (which is entirely submarine based).
This is the most authoritative source I can find right now; it's not a subject where it's easy to find a ton of verifiable information: https://ukdefencejournal.org.uk/trident-what-are-the-facts/
EDIT: For interest, the justification in Britain for leaving "full operational control" in the hands of submarine commanders is that the UK government is likely to be vaporized almost immediately after a hostile launch, there being such a short flight time from many hostile nations to the UK. The USA has more time to react, and a number of scares helped to swing political opinion in favor of mandatory PALs for submarines. Now, I have heard that there is in fact a way for a US SSBN crew to launch independently in the event of a verified destruction of the chain of command on shore, but as that verification is ultimately down to the command crew's judgement, it renders the PAL moot. This I could only find Quora answers about so take it with a pinch.
"Last strike" is arguably a necessary component of a submarine deterrent since it means even a surprise decapitation strike carries the risk of a retaliation. It's certainly why the UK delegates launch authority to the captain (again, after a procedural verification of the destruction of the chain of command).
God, this stuff is frightening.
The solution, as in Doctor Strangelove, is to take control out of your own hands - you put yourself in a position where you can say "I would never launch nuclear weapons, heavens no - but I can't speak for the commanders of my nuclear subs if their country is obliterated in a first strike. Who knows what they might do..."
I'm rather disappointed that Jeremy Corbyn doesn't seem to have appreciated this fact, as he is currently being raked over the coals unnecessarily for his entirely sensible public reluctance to murder millions of people. In fact you don't want someone who looks trigger-happy, as that incentivizes a first strike against you.
http://www.theregister.co.uk/2017/06/01/trident_nuclear_dete...
It's probably not totally crazy that one day almost the entire UK NHS or even the US election could be susceptible to a cyber attack, then we'd be really screwed.