I presume that the Trident missiles we have still work, that is they can carry out their intended function if called upon.
Also I would expect that the submarines carrying them can get from A to B or close enough to the launch location.
Then why are we spending billions renewing them when we could keep what we have and spend the cash elsewhere?
Is this just a case of the mobile phone world taking over, the need to have the latest iTrident and we need a shiny new iSub to carry them in !?!?