I have yet to see anyone making a strong case for the UK retaining a continually at-sea ICBM deterrent program.
It will not deter terrorists, now or in the future. The only conceivable use for it would be to attack another state - but can you ever imagine the UK acting unilaterally in that situation?
Its hugely expensive and now outmoded. The only people who really support it are the Royal Navy and it means, as someone else has already said, that we retain our permanent seat on the UN Security Council. Given the age of austerity and cuts in public spending, mothballing Trident could save some serious money....