Our nuclear deterrent lacks the capability to be used as a first strike weapon against a major power, but could inflict very serious damage on one, in terms of cities destroyed, infrastructure wrecked and millions of mostly civilian deaths. Most governments want a functioning state and a population to rule, so the threat of thermonuclear heck might put them off nuking us first. That's why it's called a deterrent.
It could be used as a first strike weapon against a small nuclear power such as North Korea, Iran (in the future), etc, although politically and morally we would then be pariahs. Would it deter a fanatical government from nuking us? The answer might be "probably". Even fanatics want to know they are dying for a reason and the total obliteration of their cause/country might be off-putting.
Is it worth it? If you are going to have a nuclear deterrent, Trident is the Rolls Royce option, you get what you pay for. In this case a virtually undetectable submarine and missiles that can strike anywhere on the planet with a high degree of accuracy. There are cheaper options but they all have limitations.
If you think the UK should have nukes then Trident is as good as it gets, if not, then no system is suitable.