There is a massive geo-strategic infrastructure in place to reduce the chances of someone even being able to attempt a strike like this. And we haven't even begun to address the question of why Russia would attempt such a strike. The real world conditions for your proposed scenario don't exist. Russian technological and military capability isn't there either.
Ok. We all know, that nobody wants nuclear war (or even limited conflict). There are no, and can not be any "realistic scenario" of a nuclear war. So, we need "unrealistic" one - a "perfect storm" of misinformation, stupidity, ignorance, incompetence, avarice and treason (as most of wars in the real world are started). We don't discuss about "reality", we discuss about possible scenario for Fedor Berezin's , Andy Farman's or Tom Clancy's book "The Windsors: The Last Blood"
So, once upon a time, in the Year of our Lord 2022, process of "deglobalisation" had been continued. Usage of dollar as weapon by political leaders significally decreased its importance as international money, caused a sort of global economical crisis. Everybody are searching for new possibilities, and ready for more and more risky steps.
Goals of sides: USA need to convert their military superiority over China into economical (by some sort of local conflic), but they need to prevent Russia joining the game on the China's side. They have not a carrot, so, they want to distract Russia by some sort of European war, and ready to use any good chance for it. But they, obviously do not want to join full-scale nuclear war against Russia.
EU: they need Russian resources, they are ready to join anybody in succesful "Drung nach Osten", but preffer peaceful trade. Also they need to punish the UK for Brexit to avoid futher fragmentation.
UK: economical war against EU is going to transform in local conflicts in Ireland and Scotland and deep economical crisis. They need a miracle and are ready for very risky games. They believe in their own propaganda about economical collapse in Russia and have no even a minimal understanding of the Russian (as well as EU and US), military and economical reality. Their goal - to crush "Putin's regime" (mostly by diplomatical and economical measures) and with the laurels of dragonslayers restore their leadership in the West world.
So, a small group of conspirators in the UK government (slightely pushed by Americans) decide to start the third season of "The Russian Poisoners" (first two were with Litvinenko and Skripals), but now with secret (as they think) biological weapon.
But something goes wrong: outbreak of plague had killed big part of Royal Family, big part of Parliament and thousands of ordinary citisens, but unproven blames of Russia were not succesful, and leaders of EU, USA and Russia knew, that it was not Russian weapon (even if it was artificially modificated strain of Kemerovo's fever), and demanded proper investigation. So, King Henry raised bets by the declaring war to Russia, waiting other NATO countries to join him in his crusade, and expecting anti-Putin coup or rebellion in Russia. He have only one Vanguard in the sea, two others are not ready (they are still repairing), his decision is not approved by the remains of the Parliament and the Government yet.
Meanwhile, Russia secretly transferred four MiG-31K in Kaliningrad region and ordered their hunting submarines to search Vanguard and keep ear on her.
Right before Harry's declaration of war, Putin had close conversations with USA and EU leaders suggesting them very simple choice: to join Brits in their stupidity and die, or to be neutral and live and may be even take part in the post-war restoration of the former UK.
The range of Kinjals is near two thousands cliks, so they can hit HMNB Clyde even from Russian airspace or from Baltic sea. Same way, Iskander-K and "Land Calibres" can do it from Kaliningrad's region.
Stage one: one pair of MiG-31K attack HMNB Clyde with two nukes, another pair hit buildings of MOD and HMG.
Vanguard launch her Tridents as quick as possible, right after launch of last mussile she sunk by Russian attack submarine. All her warheads intercepted by A-235.
Stage two: Vanguard is killed by Russian hunter submarine. Few Tu-22M3M with four X-47M2 each kill SAM sites and radars. Iskander-K and Land Calibers hit main military bases.
Stage three: Comrade Corbyn declare raisong of British Respublican Army to fight corrupted Windsors and their cronies. Russia support them and establish new Government.
Mod Edit: Warning issued. DefenceTalk is an international defence forum for the discussion of serious defence issues. It is NOT a place for discussion of fantasy, science fiction, or conspiracy scenarios. If someone desires to engage in such discussion, there are other venues to do so. Raise the quality of your engagement and read the Forum Rules before posting again. Continuing to bring up such fanciful scenarios which read like the synopsis of a B-grade (or worse) action/thriller movie plot will quickly led to sanctions.
-Preceptor