I really don't get the nuclear launch argument. If we were to "first strike" the opposing country would retaliate and that would spell the end of our little island. But hey at least we kill millions of innocents before we die!
Having nuclear weapons just makes us more of a target, it's not a deterrent.
I'm completely against the first strike option and agree the path, as Corbyn said, should be a nuclear free world.
However, whilst we are renewing Trident (which I am wholly against) at great expense, Corbyn should have said he would be prepared to use it in retaliation to an attack on the UK. It definitely doesn't act as a deterrent if your enemies know you won't be prepared to use it. He should have just lied.
I do think the chances of a nuclear strike are minuscule though. Much lower than @MikeUpNorth's 5%. Cyber warfare is the future. No one is going to fire a nuclear weapon. In doing so the aggressor is basically wiping out their own country.