Total horseshit.
Sarmat was never successfully tested, and even if it did carry sixteen 550 kT warheads, and we assume FAR greater accuracy than we've ever seen from previous Russian ICBMs, that's just shy of 9 megatons.
That's peanuts opposite the largest nukes ever tested which *combined*, wouldn't be enough to "bury the UK under the sea."
So, unless someone has mistranslated Putin's words, I say he's just stirring the pot, probably as a red herring for how poorly the Ruskies are faring in the Ukraine war.
Besides which, that's all just saber-rattling. Vlad knows good and damned well that whatever imaginary tactical gain he might enjoy with some ostensible wunderwaffe will be strategically countered, smashed and humiliated by NATO. He ain't gonna do shit to us.