The dangers to any non-terrorist, state based nuclear strike is most certainly a very big deterrent. Maybe not for a suicidal dictator himself but for all the military and engineers in the chain of command and their families and homes. I would imagine what we saw in Israel is a small reminder that defensive technologies are actually more powerful than offensive ones so you can bet on many layers of defense. That said, I can’t picture a global nuclear winter because the jet stream is not some uniform air flow but highly channeled. So if a series of foolish mistakes took place, the Southern hemisphere would probably inherit the polluted North. Remember, a single hurricane, tornado or earthquake can wield the power of many nuclear weapons. The smaller terrorist type attack is awful but not an extinction event. Also we have many more existential hurdles to get by such as bio weapons and nano bots and eventually the nuclear specter will fade because fissile materials will be neutralized with newer methods. I’m not advocating for trying nuclear war but think we can get by without a large scale event. And no, Russia would never dare attack London under any circumstance but would definitely make people think they would. Psyops and brinkmanship are their favorite games. See, it’s working.