In a World War II in which the Soviets and Americans never get involved in, how can the Allies (Britain, France, and lesser-known countries) win WWII against the Axis?
The French agree to the Franco British union.
Said union slowly overwhelms Germany by sheer weight (2 million Indian volunteers went unused OTL). Alternatively, they hold the line until tube alloys comes online.
This, basically. If things don't change until 1941 so that Britain has no allies left on the European mainland then it's a matter of holding on until Tube Alloys delivers a nuke, probably in the 1946/47 time frame.
The two million Indians didn't go unused BTW - a lot of them were required for internal security. Probably the only easily redeployable troops would be the half a million or so men of 14th Army if Japan stays out of the war which consisted of two British, nine Indian and three African (81st and 82nd West African, and 11th East African - all famously tough units with exemplary combat records) divisions. These should be enough to secure the Middle East and clear North Africa even without the US in the war, but after that you're in a classic whale vs. elephant scenario until a game changer happens, and for a British/Allied victory that is most likely going to be nukes.
Yes in theory but practically no.
Germany's operative revolution that was quickly summarized as blitzkrieg was a massive advantage that could have been contained only with an enormous amount of luck or with a much bigger numerical advantage.
In a World War II in which the Soviets and Americans never get involved in, how can the Allies (Britain, France, and lesser-known countries) win WWII against the Axis?
Well not really, if the Allies had acted on any of the mountain of intelligence they had about the attack on the Ardennes or if the French had been able to moun their part of the counterattack at Arras thing scould have been quite different. And of course if Hitler had gotten his way Germany would have attacked in the Autumn of 39' before Sickle Cut was on the table.
... Ultimately Churchill's hold on the English is greater than Hitler's hold on the Germans.
...
I'm inclined to agree France was likely to lose over the immediate run - but the battle of France was quite spectacular as a failure.
If the French don't do the exact worst thing (deploy the best, most mobile forces such that they can be cut off through the Ardennes, without defending the Ardennes properly), they'll instead have a series of running battles that grind down the French but also German forces. At that point the question is if the disparity in effectiveness (in Germany's favour) is enough to offset the disparity in economy (in the Allies' favour). It's likely to extend the battle of France into autumn if not winter, IMO. And without such a clear 'we are absolutely beaten' feeling as developed after the second French line was overrun, there might never be a Vichy, so you could have the North African theater as a quick mopping up of Libya from two sides, followed by a massive buildup in both Britain and Tunisia, so that the Allies can snipe away at the edges (Sicily, Sardinia?)
One thing a slow grind might also ensure is Italy sitting tight much longer, or engaging in Greek adventures instead (massively helping the British economy as ships can keep going through the Med).