The UK has four Vanguard class missile submarines in various states of readiness – generally only one is available to patrol at a time. They carry 16 American Trident II D-5 missiles, each with up to fourteen warheads varying in size from 7kt to about 500kt. The larger warhead could destroy a large city like Moscow.
From their base at HMNB Clyde west of Glascow, they can reach almost all of Russia and parts of China. To reach the entire potential target area (China, Russia, Iran) they presumably would have to at least patrol from the Indian Ocean or the Pacific.
The Trident nuclear missiles can carry weapons of up to 475 kilotons, meaning that you get 3rd degree burns at a distance of just below 8km. If detonated over central St. Petersburg, some 390,000 people will die immediately, or up to 600,000 if it’s set to detonate a couple of km up – this will be less than the Siege of Leningrad.
The range is pretty much immaterial. They are ICBMs, so they can reach all of Russia.
No one knows where they are at the moment—not even the British. Only a few senior submarine officers know their exact location, this makes it totally impossible for Russia or other country to take them out.
It’s a fair guess it’s somewhere in the Arctic, where they can reach major population centers and Russia’s nuclear launch facilities. This gives a flight time of a few minutes; the retaliation can be on its way before a Russian nuke even has time to detonate.
