I am not against the replacement of the Trident nuclear deterrent on moral grounds. Work (Cray and Sun) took me through the gates of the Bomb factory at Aldermaston a couple of time about 10 years ago. I would rather such weapons did not exist, but in the world we live it can be argued that such weapons are a tool to do a job.
The problem today is that UK PLC faces greater risks of destruction from inside. The mains ones are economic and social. No sane country is going to invade an other country which owes 35,000 pounds per head. Terrorist risks are far lower level and Trident next does not deter that threat.
Why Trident is far less relevant today is well articulated here.
So why might you want a nuclear capability? The only good reason I can come up with is the low probability, but very high impact that the earth would be hit by a large meteorite for which Trident next would be of little use to help it change course.
From reading around the subject, it appears that the missiles and warheads can continue in service, but it is the submarines which have a limited lifespan which is coming to an end.
Trident next is a luxury for a country which has a budget surplus. Until that time, as a country we need to make do with that we have or go without. A nuclear capability on tick makes no sense at all.