All right, here's another AH challenge: OTL, Finland joined the Axis as security against the Soviet Union. How would you get Finland to join the Allies, and isolate the Soviet Union?
Would be tricky. Finland also has a border with Norway, which after April 1940 was occupied by the Germans, so the Finns would be vulnerable, and NO WAY are they letting the Soviets in to help defend Finland!
However, the following is one way. (Everything below is OTL, not made up by me.) Unfortunately, it probably involves Britain and France losing the war.
At the start of WW2 Churchill was brought back into the government and went to the Admiralty. When the Soviets, who at the time were the nazis notional allies, invaded Finland, Churchill was desperate to launch an operation to assist the Finns.
So he had a brainwave! We couldn't get to Finland or the Soviet Union, but he came up with a plan to seize Sweden's iron ore mines, partly to cut off Nazi Germany's supplies and partly to prevent the Russians getting their hands on them (there's no evidence that the Russians ever considered invading Sweden).
The only way to get to the iron ore mines was to land in Norway. So Churchill planned an operation where the Allies would land on the Norwegian coast, march across country and invade Sweden, seizing the mines.
Because there was so much argument in London about invading two neutral countries, before the operation planning could be finalised, the Finns surrendered. If we had launched the invasion in time, Finland might very well have joined the Allies. But Norway and Sweden, and possibly the Soviet Union, join the Axis.
Which not only makes us look as bad as the Nazis and the Soviets, also means we never get our hands on the Norwegian Merchant Navy, which made a huge difference in the Battle of the Atlantic.
(This to me is the best chance of an ATL that wins Germany the war.)
But it gets better. Once the Finns have surrendered, the planning is so far along that the Allies decide to invade Norway anyway!!!

On the 8th April, the RAF dropped mines in Norwegian waters. The following day troops were being loaded onto RN ships to sail later that day, when the news came through that the Germans had invaded first. Liddell Hart called the race to invade Norway 'a photo finish'
If the Germans had delayed their invasion by as much as a day, or hadn't felt it necessary, we would have looked as bad as them to the rest of the world, brought half of Scandinavia into the war against us, and lost Churchill from the government only a month before he becomes Prime Minister (it's a huge irony that he became PM after the Chamberlain government fell over the failure of an operation that was Churchill's idea).
Incidentally, I don't see the Allies defeating Germany in Norway in 1940. At that stage of the war, they were simply better at fighting then we were, because they had more experience of how to run a wartime army. Ours was still thinking like a peacetime army.