When the Labour government of the 1950s (oh, wait, was it "the Left" who started this all?) initially started ordering Nuclear Weapons the arguments were far less about defence and far more about leverage with the Americans. This argument quickly got lost as the US ended up supplying us with the weapons in the end so we became dependent on them... but I think the point holds that Nuclear Weapons is often a political rather than a strategic point. There is fundamentally no situation in the future where either side is likely to fire a Nuclear Weapon, as long as their targets' allies also can retaliate on their behalf. I'm not convinced from a strategic point of view, then, that we need weapons if the US have them.
Nor indeed is it clear any more that it does provide leverage. It didn't then and it doesn't now. Economic ties, or severing them, are often far more of a threat.
In the meantime, a more real threat perhaps is not some Nation state launching weapons as an act of War, but of terrorists getting hold of weapons and launching them from a place against which it is impossible in principle to retaliate. Again, what then is the point? You can't fire back against a moving target.
I think the effort would be better spent on defence systems, rather than aggressive ones. If the only tie we would consider firing a Nuclear weapon is if the other side did first, we may as well put some effort into stopping their weapons from getting through instead since that would be almost as effective in ensuring safety.
In terms of the "Argument from Authority" -- greater minds than ours have also rejected the case for a Nuclear deterrent. There is division.
Something that has often puzzled me slightly, by the way -- the argument from deterrent should work both ways, no? If your enemy has them then so also should you. So... why do you think Iran might be so keen on developing Nuclear Weapons technology? Perhaps it is in part because their perceived enemies in "the West" also have such technology. Just a thought.