The Japs were after resources that they needed badly. The "embargo" that the USA had on them meant a choice between war or some submission, - which doesn't exactly suit a Samurai.
With the UK out, at peace with Axis, Japan would have had it's trade and resources within the British resources. Burma. Malasya. British parts of China.
The USA could not muster a power to stop that, and the only foe of Japan apart from China would be...USSR. Time for revenge after the 1937 (or was it 1938) disaster.
And Lazs, you didn't take the load. IMHO the Russians did, and as a relative thing, the British.
The British took the heading course in the summer of 1940. So this is just a speculation what would have happened if they hadn't.
Now, back to Barabarossa.
Someone said that the USSR would have been prepared much better had Britain been out. While the USSR would then only have to worry about Germany, they only had to worry about Germany! In my "scenario" They would also have to worry about Japan! On a funny side, the USA was USSR's big Ally in the far east by keeping Japan tied up on their resources, and eventually getting into a state of war.
Germany would have been much faster as well as much stronger (as Nash has pointed out regarding the LW, although I only guessed at losses from&with BoB + the med + the western fron untill spring 1941, - carefully!!). You see, lingering on with some plans of invading Britain in 1940, and testing the UK resolve took quite some time. Instead of turning to the USSR maps in July 1940, Hitler firstly did so in December. So I'd rather discard the theory of the USSR being prepped because of UK stepping out, - I'd put the scale on trouble brewing up in the east as well as Hitler being much faster and much stronger.
Then on to the airforces. The numbers didn't tell much. The USSR had numbers, but lacked quality, training, experience and tactics.
The LW had those, but needed more numbers. Yet, with what they had, they really gave the red airforce a bloody nose. Even in 1943 or 1944 the huge red airforce never even closed in on the LW's losses in the med and the W-front. So, I have little doubt who would have been the ABSOLUTE king of the air with thousands of extra aircraft and expert crew over Russia, - if not just in 1941, then in a prolonged war.