You know why the huge 50 - 100mega-tonne bombs were developed? So you could drop one on a population centre and destroy the industrial capabilities without having to have good intelligence. Drop one on London and you instantly have ~5 million deaths.
But of course if you drop one on London Britain launches back at you, which means you have 16 - 32 missiles coming back at possibly your population centres (Because your first strike is at the Clyde for obvious reasons, you don't have the full number of missiles)
Plus, NATO would also have to launch. France probably would as a 100 mega ton bomb on London would cause major issues for France if the wind was in the right direction. In any case, you seriously annoy the EU.
Lets say it came from the US - who, as far as I understand at the moment, hold 'space' superiority at the moment. As well as a massive anti-ICBM grid over the entire country.
Say they dropped it on London, (and it actually made it there) it would cause issues for France, etc., but if you were going to do that, you'd probably predict that and just point another one at them to 'persuade' them from even trying to retaliate in any meaningful way. Off the top of my head the US has 1200~ nukes (not sure after they decommissioned some), which is
more than enough to dissuade any attacks from other countries. Seriously, who wants to be the one that pokes the US enough for them to even look at using a first strike nuke? Add onto the anti-ICBM grid, the US has by
far the largest Air Superiority, nobody would even be able to get to them without getting dropped.
In other words, use a nuclear weapon and you have a death toll that make world war two look like a walk in the park.
It is game over for you and the other side and your reputation.
Not necessarily (on the death toll part). There were far more casualties in WWII from normal warfare than there was from the two nukes dropped on Japan - lets face it, dropping a nuke on someone would cripple them to the point where they don't really have a realistic chance of fighting back, especially if they're sitting there thinking "when/where is the next one going to hit?"
And if you're going to use a nuke, I don't think you're going to give a damn about your reputation by that point.
I'm not saying use it as a 1st strike because you suddenly decided that you wanted more land under your control (because nuked land would be pretty damn worthless after a blast of that magnitude), I'm saying if you were provoked by something serious enough to lead to a full-scale war. If you're going to go to war, why not end if before it's even started?