"You haven't thought of the option of nuke-tipped TLAM from Astute Class. I think this would further disperse our nuclear forces if needed. They already carry TLAM and so this would be a no-brainer for me. You would now have free-fall and submarine based deterrence."
Its an absolute no brainer for the simple reason that we'd have to develop on our own a nuclear warhead to put on TLAM and then ensure we can plan the missions, execute them and that sufficient numbers of missiles can make it through to actually deliver the desired effect. It is vastly vastly more expensive than the current setup.