To bring this country to its knees, you do not need to bomb us into surrender, or invade us. The way to do it was clearly demonstrated in both World wars: cut off our sea supply lifeline through the Western Approaches. It was just our good fortune that both the Kaiser and Hitler were land animals, and did not divert sufficient resources into their U-boat campaigns to do so.
This danger had long been known. A century ago, Kipling put it into words in his poem "Big Steamers":
"The sweets that you suck, and the joints that you carve
_Are brought to you daily by all us Big Steamers
_And if anyone hinders our coming, you'll starve".
And, not for nothing, did the Victorians urge: "RULE, Britannia, RULE the Waves !"
In mid 1941, despite the heroic efforts of the RN and the (often forgotten) merchant seamen, tonnage coming into the UK was being sunk faster than it could be replaced. There is only one end to this. In his memoirs, The Second World War, Volume 2, published in 1949, Winston Churchill confessed: "The only thing that ever really frightened me during the war was the U-boat peril".
As it was, the Navy and Coastal Command got the upper hand at last (but would they have done so without "Ultra" ?) No doubt Putin's Naval Staff Colleges are giving serious attention to this ..... (Why look in the Crystal Ball, when you can read the book ?)