By World War I, the British had moved to a strategy of distant blockade (very effective against Germany in WWI, BTW). They could, in theory, have controlled trade in the Baltic by controlling the North Sea. Similarly, the Mediterranean Fleet could have controlled the Aegean entrance to the Black Sea, and control of the Greenland-Iceland-UK Gap could shut down access to the ports in northwestern Russia.
The closure of Vladivostok in the east would have been quite a poser, and I’m not sure the Royal Navy could have done it without the cooperation of the Japanese, but also remember that the USSR and Japan had fought set-piece battles as recently as 1938 and 1939. There was not a lot of trade between Japan and the USSR, so I doubt the IJN would have pulled out to help the Russians. Vladivostok is pretty easily sealed off in theory, ships having to pass through straits easily controlled by Japan to get there. To be honest, though, considerable economic crippling of Russia was accomplished in 1914-1917 with just the closure of the Baltic and Black Seas, and Vladivostok left wide open.
In any case, your thoughts on the political situation are right, it wouldn’t have made sense for Britain to try to economically cripple Russia with a blockade, although perhaps they could have.
Uh Oh… I thought we were in GQ! :eek: