Forgive me for not being very reassured by thatLike you said, the effects would be devastating and billions would likely die.
I stand by my statement that only a madman would give the order for a nuclear first strike (esp against another nuclear power). I'm not even sure if Jim Jong is deranged enough (maybe he is...?)
So whilst May might say she would (talk is cheap), I would hope it's just a bluff.
Agreed only a madman would - it is kind of the point of the MAD doctrine though that no one can be sure if its bluff or not so why Corbyn significantly compromises it (and I suspect done on purpose due to his misguided idea that you can shortcut to fixing a system by absolutely breaking it without regard for the consequences).
North Korea is a different case - the siege mentality of their leadership mixed with paranoia means that they likely would strike first if they thought there was a way to do so while minimising return damage - while most other countries even Russia would be very unlikely to do so.