Replacing Trident

This is as much a profession of ignorance as anything else, I realise. But the thing is, I entirely see the old Cold War logic of deterrent nuclear missiles in submarines, targeted against an obvious, similarly armed potential foe, namely the Soviet Union. The submarines were supposedly difficult to find and destroy. In the event of an attack on the UK, which might even destroy all major cities and government institutions, a counter strike was still possible even if pointless, thus creating deterrence by mutually assured destruction.
However, I don’t understand the logic of replacing Trident with a similar setup, even though both our major national parties are currently committed to it. Whom are we deterring? At whom are the missiles to be aimed and potentially fired? If there were a crippling attack, presumably disrupting communications and the chain of command, what would be the submarine captains’ instructions? How would they know what to do?
Is this a classic example of fighting the last war, even if in this case the Cold War? Leaving aside the obvious observation that it is difficult to deter a suicide bomber by threats of destruction, the uncertainties are far too great for the deterrent strategy which worked in the Cold War to work today against terrorists and rogue states. So what are we doing trying to repeat it? Even if the greatest threat is deemed to come from a major power, which one is it and how does the isolated submarine captain know at the relevant time? If it is country X, what is to stop the enemies of country X launching a nuclear strike on the UK in the happy expectation that our submariners will launch their missiles?
Thus we could be made less not more secure by deterrence in modern conditions. Even if nuclear weapons are actually deemed essential options for our security rather than (as one sometimes suspects) the international prestige of our politicians, surely the strategy of missiles under the ocean cut off from outside interference is no longer relevant.
What are we doing? It’s not as if we couldn’t think of anything else to do with the cash, even if it were spent on other weapons. But the political class dare not voice such thoughts because to do so might be attacked as “soft”. So we blunder along into another expensive defence mistake. Plus ca change.