The only realistic threat is a rogue launch from the likes of Russia or a terrorist attack, neither of which would be deterred by Trident (or any of the other systems people have proposed, for that matter). The entire concept of deterrence is deeply flawed and a throwback to a bygone era.
you have answered your own question!
As already stated, Trident may be be used in retaliation - rather than a deterrant.
TBH, If we have gone nuke in retaliation, then it's probably the end of civilization as we know it. In that situation, I wouldnt give a monkeys about anything and would be glugging that special bottle of Bacardi Reserve i'm keeping for some unspecified occasion.
Do you really think politicos of any party (fringe loonies aside) would have the balls to press the button?