Henra / Willard: I've never claimed that this offers the same level of capability/survivability as CASD Trident. But that's not the exam question here, which is "what's the cheapest way of delivering credible minimum deterrence (so that we can spend the saved cash on the conventional forces)?"
If money were no object, then 4-boat CASD would be uncontentious. It is, so it isn't.
Cornish-Stormrider: it's a good book, referenced at notes 107 & 109, actually. Brave boys indeed at Damascus. But the UK forces' nuclear safety record has been good, so what makes you think it will get worse now?