Trident is a tricky issue...
I would love to see the day where a nuclear deterrent was "clearly" not needed. Unfortunately, I don't think we're really there yet. In 20, 30 years I really hope we are, but for the time being my *personal opinion* is that we should keep the nuclear deterrent.
I pay (what feels like) a fair whack of tax every year. The system costs ~£20Bn, and with 26million taxpayers that works out at around £770 per taxpayer. For me, given the global situation, it's still a worthwhile investment (for now). Consider that this is a tiny percentage of the money spent on bailing out the banks in 2008 / 2009 (~£955Bn), and a relatively small percentage of our national defense budget (~£45Bn per year).
Regarding Cameron's statement... Its language is clumsy and imprecise, yes. But let's be realistic: If North Korea continues its development unchecked, then it will have the capability to strike anywhere in the world with its long range missiles within a decade or so. The decision on whether or not to renew Triton spans well beyond that time-frame, so it's something that must be considered.
It's a cliche, but in this case I'd say "better to have it and not need it, than to need it and not have it". The trident nuclear deterrent allows us a certain amount of military independence from major world powers, and it's hard to put a price on that.