Dmitry Kiselyov, Russian state television executive and propagandist, in addition to threatening the UK with a Sarmat missile known as Satan II and “will be enough to sink it once and for all,” has now threatened that the use of one of Russia’s Poseidon torpedoes “could "plunge Britain into depths of the sea,” as “"The explosion of this thermonuclear torpedo near Britain's coastline will cause a gigantic tsunami up to 500 meters high.” Apparently, the torpedo “approaches its target at a depth of 1km at a speed of 200km per hour. There's no way of stopping this underwater drone.”
As the weapon is supposedly more than 24 meters long, it's probably more accurate to describe it as an unmanned submarine than as either a drone or a torpedo.
It is presently untested. If it travels at speeds of 200 kph (or over 100 knots), it would need to be rocket-powered. it is designed to be carried by a submarine in a sealed tube. Russia currently has somewhere between 49 and 64 submarines, of which it has only one special purpose submarine from which Poseidons would be launched - the B-90 Sarov. Its reported test depth is 300 metres (980 ft).
Way to stop them 1: Seed the seabed around the UK with networks of sensor-mines to detect and destroy them. The seabed around the UK is comparatively shallow, i.e., nowhere near 1000 meters deep.
Way to stop them 2: Use submarine-launched long-range hyper-sonic glide vehicles with payloads that could be lightweight torpedo or nuclear depth charge.
American underwater nuclear explosive tests in shallow waters (admittedly at a lesser kilo-tonnage and not discounting that damage and long-term effects caused) suggest that generating a "tsunami" that “could "plunge Britain into depths of the sea” is more Russian bombast, i.e., high-sounding language with little meaning, used to impress people - most probably the Russian people.