I've thought a lot about this, on and off, over the past decades. One has to question what purpose, actually, does Trident in its present form now serve? To protect against a nuclear attack from the Soviet Union that ceased to exist? Or from Russia? And in which case in what circumstance would a nuclear attack on Britain by Russia go without a response from the USA by nuclear retaliation?
So does it now exist to warn a "rogue" state that a nuclear device that originates from there, and is exploded in London will result in a nuclear attack on them? OK. Except this could be done by cruise missile more easily than by ballistic missile...simply because if the ballistic missile passes anywhere near Russia what do you think they will interpret it as? Yes..a possible nuclear attack on them.
Look at a map of the world. These things don't fly in a straight ballistic curve line per se but go over the polar route so any nuclear ballistic missile fired by a British Trident against what North Korea? Iran? Draw a line on the map. See who it passes over from where it would be launched from. Russia and if Korea is involved possibly China as well. It isn't going to happen!
Does anyone really think either are going to sit back and just track a nuclear ballistic missile heading over their territory? They most likely go on a full nuclear alert and the USA on seeing that would also do so. So Britain has now pushed the world onto the cliff edge of a nuclear war.
So a cruise missile, nuclear armed, is easier to launch (in terms of reaction from Russia and China), and easier to programme to target in that it can safely be routed to avoid them other nuclear armed countries. Indeed it would be liley fired from a Royal Navy vessel...ship or submarine...laying somewhere off the coast of Korea or the Persian Gulf.
No I think that Trident is now irrelevant to the UK....nuclear armed cruise missiles still have a purpose. Either air, land or ship launched. But Trident? No.