Nuclear weapons get a lot of flak because they're dangerous, and a bit pricey. Both of these things are true. However, people getting upset about them because they watched Threads once and it (rightfully - very disturbing TV feature) scared them detracts from a rational debate. This debate would be over what nuclear weapons have achieved and continue to achieve (if anything), and whether or not Great Britain should keep or get rid of them - which would equate to unilateral disarmament.
People who want to get rid of nuclear weapons in the UK normally have one of two arguments. The first is that nuclear are bad and that we should get them because we should not, on principle, build nasty weapons of mass destruction; we could be spending that money on schools and stuff. The second is that nuclear weapons are ridiculous because they will never get used by Britain.
I am not unsympathetic to the disdain of the hideous destructive force wielded by nuclear weapons, or to the notion that we should live in world where there is no need for them. Ideally we would not spend money on these weapons, and would live in harmony under one government and would have no fear of that sort of nastiness. However, nuclear weapons exist.
Separate governments exist at least in the foreseeable future. But importantly, no mechanism for keeping them from tearing each other apart exists, aside from nuclear weapons.
World War Two ended with two nuclear detonations. Since then, international diplomacy has not been removed from the stage of nuclear politics. Since the end of World War II, in all conflicts worldwide, around 55 million people have died, most of which have been in civil wars. More than that number almost certainly died in World War II alone, and the global population has almost doubled in that time.
This is in spite of the fact that weaponry has evolved and become more deadly. Jet planes, tanks, rifles, missiles, and warships have all become exponentially more effective since then. No major industrialised countries have directly gone to war with each other since then; there has not been anything close to the scale of the Second World War.
Whilst a counter-factual history without nuclear weapons is difficult to comprehend and impossible to predict, it is reasonable to observe that the major mechanism in preventing large-scale armed conflict in the world has been the presence of nuclear weapons, and the doctrine of mutually assured destruction.
The Soviet Union and the United States could not settle their disputes on the battlefield, and so they were forced to resort to negotiating and not wasting resources (except perhaps those on the big scary missiles). Nuclear weapons have removed the possibility of using war, or its threat as a foreign policy instrument against certain powers. At the same time, the balance of power in the world, where a few big to medium sized powers possess nuclear weapons, mean none can feasibly use the nuclear weapons without reprisal.
This is a delicate equilibrium, yes. However, given that before this, industrialised countries seemed to go to war with each other every 20 years or so, wielding more and more destruction each time, it seems preferable. The world has effectively traded potential ceaseless unconstrained carnage, for a much smaller potential of total oblivion. Any argument for the worldwide scrapping of nuclear weapons, would have consider a return the status quo beforehand, where increasingly destructive war is a very real potential. Whilst wars have been fought all over the world whilst nuclear weapons have existed, they have either been internal conflicts, or small, regional conflicts which have either been fought by professional soldiers or partisans.
Therefore, short of some miracle mechanism that creates world peace, even multilateral disarmament does not create a safer world in the near-term. Until global institutions mature significantly it will not be possible to guarantee the level of stability that nuclear weapons grant.
Turning to unilateral disarmament, the common argument that spending loads of money on nuclear weapons is dumb because we'll never use them is baulk. To suggest that requires two abilities that no one on this or any other Earth possesses. A: foresight, and B: an ability to accurately model counterfactual futures.
Firstly, no one can predict the future. The world is mental. No one ever ten years ago could have predicted the precise state of the world now. No one 25 years ago would have even come close. No one 50 years ago would have had anything but vague insights to offer. (See: Orwell, Huxley) The future is rabidly uncertain, and nuclear weapons are a hedge, and defence against uncertain.
Secondly, anyone who suggests they will never be used is missing the point. They're not meant to be used, they're a deterrent. Now, if I correct this to something conducive to an argument, that they would never be useful, then we can analyse exactly how ridiculous this line of 'reasoning' is.
To suggest nuclear weapons would never be useful in self defence, requires us to imagine a world in which Britain doesn't have nuclear weapons, and then honestly say that in the next 40 years, Britain will not be involved in security standoff of some-kind with a nuclear armed power, that could threaten it with force of any variety, to which the UK would have no means to defend itself. No one has the ability to say that with even a single percentage, not even a single basis-point, of certainty.
No nuclear weapons do not provide much protection against terrorism. No, they have nothing to do with stopping global warming. But we can be pretty certain no one's tanks are going to rolling through Cheshire anytime soon. We can also be reasonably certain no one is going to order a nuclear strike against the UK as long as we have nuclear weapons, because of the costs of doing so.
Who would do such a thing? Right now - who knows? In even 10 years? Who knows? No one.
This does not mean we should remain religiously wedded to Trident, or any launch system. We should not even be wedded to nuclear weapons in particular, if we can find a more efficient or humane way of responding to an attack against our state and its people that would serve the same purpose, then by all means we should endorse that alternative. But the current state of the world requires that to ensure to the best possible security of a state, that it possess weapons of mass destruction. In addition, given the present state of the world compared to its predecessors, it would not be incorrect to suggest that they have made it a safer place.
This is perhaps depressing, the notion that humanity requires a threat of destruction to behave itself collectively. However, it is within the peace that results that we can find optimism; for example the hope that was ignited by the images of the collapse of the Berlin Wall would not have been possible had the Soviets and the United States knocked it down in a prolonged tank battle in the 50s.