How about the a/c carrying the German plans isn't lost over Belgium. Hence the Germans carry out a more traditional attack through the low countries, met head on by the best allied units. Superior tactics and co-ordination with the Luftwaffe means they win out but its a very bloody 3-4 month campaign with much higher losses on all sides before France surrenders.
The heavy casualties, possibly with much of the BEF being taken prisoner means that Halifax [or ANOther] becomes PM instead of Churchill and Britain makes peace. Possibly after a period of skirmishing that persuades the Germans an attack on Britain is not going to work.
The German attack on Russia occurs with somewhat weaker forces as Germany is still recovering from the losses the previous year. Coupled with the fact that even Stalin realises a German attack is coming, the Red Army does a little better although the Germans make huge gains. The war in the east lasts until ~45 before mutual exhaustion persuades the two powers to agree terms - possibly by this time Hitler dies of natural causes or one of the assassination attempts succeeds.
Faced with a full strength Britain as well as a France that while partly disarmed is better able to defend its interests Japan dare not drive south. Also with a weakened France and Britain wishing to avoid another costly war and the Netherlands under German domination Japan is able to buy the raw materials it needs. The war drags on in China and Japan is persuaded, for whatever reason, not to attack Russia. Finally, fearful of a reviving Soviet presence in the east and drained by the war Japan comes to terms with Chaing Kai-shek to end the conflict. It maintains control of Manchuria and neighbouring provinces and a lot of influence in China itself, although bitterly hated there.
By the late 40's the world is without major conflict but heavily militarised. The Nazi and Soviet empires glare at each other over a highly fortified border while Britain, aided by the Dominions has built up an impressive navy and air force and also considerably upgraded its industrial base. America has surpassed the RN in strength and also has a very large air force. [Or two if you count the naval one]. Japan is clearly the weakest of the 5 blocs but is also maintaining substantial forces.
The initial development of the atomic bomb in Britain or America hence occurs in peacetime when there is fear of war and a desire for efficiency savings but no actual shooting war. Intelligence leaks and/or the desire of the democracies to show what they have to deter attacks means that information about the new weapons leak out, meaning the other 3 blocs are motivated to prioritise their own research and probably get some hints about what's the best route.
By ~1955 all 5 powers have nuclear weapons, along with some capacity to deliver them. This further heightens tension but also means no power dare start anything. Especially since even if it 'won' by doing more damage to its opponent that it took itself both would be weaker compared to their 3 undamaged rivals. The world continues to be divided into 5 highly militarised blocs with mistrust and a large degree of economic isolationism. Even the Commonwealth and the US are at, relatively minor, loggerheads over the continuation of the British empire in much of Africa and US attempts to control much of Latin America.
Steve
OCC - Not saying this is a likely change of events but might happen and seems to be the best way of achieving the desired result. Also not clear how it would end but could get very nasty.