I can think, straight away, of four or five reason why this would have been very much the wrong thing to do.

• You make an enemy of your biggest allies. Nukes or no, the US has never been more powerful than the rest of the world put together.
• You don't react to coming out of one Cold War by initiating another.
• This strategy is pointless unless you plan to follow through. The regime that laid down that threat would either be strung up when they launched, or voted straight out when they didn't.
• Mutually assured destruction was what stopped nuclear war happening. Setting one country up as the Guardian of the Nukes is stupid, even if you are that country. I'm not a yank, but I believe this sort of idea is pretty big in the constitution.
• Attacking London is a shortcut to getting a pounding. This one's just conjecture.

Basically he was about ruthlessness for the good of humanity.

Yeah I think the clue is in there. Better to be about the good of humanity, and ruthless if that's what's called for. Setting yourself up as 'the guy who has the balls to make the tough decisions' usually denotes you as a nutjob. Case in point: von Neumann suggesting launching was the right strategy. I don't think anyone would argue today that he was right, though back then the decision must have seemed pretty much impossible to make.