I'm sorry, but I don't follow Cook's scenario. Nobody foresaw the catastophe in France. The Entente had decided that the invasion of Poland made clear that the Nazis could never be trusted and would never stop, and had to be rolled back by military force.
Why would their resolution falter before the military catastrophe, when Britain's held firm after (and the French did pretty well for a defeated county), without Germany offering any concessions?
Anyway, I'm not really an expert on the Soviet readiness at the time with regards to Stalin's connectedness to reality, the military equipment cycle, or the Purges and re-organisation. I can't really offer comment on the military likelihoods.
But Britain and France are unlikley to immediately start supporting the Nazis to the hilt (we had been negotiating with the Soviets before the M-R pact, and it wasn't that negotiations decisiely failed, it was just that we never made the effort to convince Stalin not to try Germany instead by giving him hard gains and military commitments), given that the Soviets, whatever their prospects look like, are unlikley to just roll to Berlin.
We might end up attacking the Soviets with bombing raids on Baku and Batum, snuggling up to Romania and Turkey (and Finland?), and trying to gain the most from the two ripping into each other, but it's not going to make us just forget how Nazist promises are worthless and their aggrandisement has to be reversed.