Operation Sealion (the German invasion of Britain) was planned for September 1940[1]. The German invasion of Russia[2] commenced June 1941.
Note that Orwell was writing this in March 1940, before it was clear that Britain would win the Battle of Britain (the airwar that was supposed to destroy the British airforce in preparation for invasion) and Germany would be unable to invade. His whole quote is worth reading:
Probably, in Hitler’s own mind, the Russo-German Pact represents no more than an alteration of time-table. The plan laid down in Mein Kampf was to smash Russia first, with the implied intention of smashing England afterwards. Now, as it has turned out, England has got to be dealt with first, because Russia was the more easily bribed of the two. But Russia’s turn will come when England is out of the picture—that, no doubt, is how Hitler sees it. Whether it will turn out that way is of course a different question.