It is rather unfair to say that Poland stood alone. Both Britain and France said that Germany was not to invade Poland under threat of being at war with them (both mostly believing they were calling Hitler’s bluff). Within two days of Germany attacking Poland, Britain, France, Australia and New Zealand declared war on Germany. Poland was the reason why these countries went to war. (Within two weeks, Russia also declared war, not against Germany but against Poland. Many in the British Empire expected Britain to declare war on Russia but that didn’t happen.)
The question of how Britain would have coped if Russia managed to stay out of the war can be matched by the equally hypothetical question of how Germany would have coped if Russia had allied themselves to Britain and France, and instead of invading Poland, had supported it. Maybe Germany would have been stopped dead in its tracks in September 1939. All excuses about not being prepared aside (and Britain was woefully unprepared for anything but policing its empire and managing the world’s oceans) Russia had a far greater military advantage in late 1939 than they did when Barbarossa was launched against them.
It is interesting to hear how WW2 is taught in Russia. Russia had no reason to expect Britain to do anything but laugh at its misfortune when attacked. Russia had, after all, been supplying the German war effort against Britain and her allies. Instead, Russian pleas for raw resources and war material were met with every effort available. If it wasn’t much, it was more than nothing and many British sailors lost their lives trying to get it to Russia, and British troops went without because equipment was diverted to Russia, including supplies from the USA that Britain had ordered for its own needs. Russia was asked for help, both in men and rocket launchers (the ‘Chatuskas’?) but help was denied, Russia claiming (fairly) that it needed everything it could produce.
No-one is pretending that the Russian forces were not huge, eventually utilising their vast country (and wonderful tanks) to devastating effect upon the invaders. But just as Russia might sneer at the efforts of little Britain in Europe, so it should recognise that its single front, while enormous, was all in one theatre of war. The 275,000 Axis prisoners of war captured in Africa in 1943 pale against the 91,000 Axis prisoners of war captured in the Stalingrad pockets only because Russia had eliminated around 200,000 Germans, while the British allied force in Tunisia fought only a quarter that number of Germans, with the rest of their enemy being mostly Italian and African troops. Russia had one huge fight, while everyone else fought a world war.
North Africa was vital to the success of the British war against Germany and for Russia, as well as the efficient operation of war for Britain in the Pacific theatre. Hitler possibly never saw it but Mussolini did, as did some German staff planners and everyone on the war cabinet in London. If Egypt fell to the Axis, it would cut the vital supply line through the Suez Canal. That would mean extra time sailing supplies and transferring forces around the globe from Europe to and from the east, Australia and New Zealand. Worse still than this logistics issue was the open door to the south of Russia. The Caucasus, with the much-needed oil fields, would have been open for attack from the south. If Italy had succeeded in taking the near east, elements of Persia would probably have joined the Axis forces; instead, they were held in check by the presence of the British. That the numbers of divisions were small is irrelevant: keeping those troops there was vital and supplying them through the Mediterranean was costly in lost shipping (one of my uncles was marooned in Africa after his ship went down; he ended up joining the army to be fed and stand a chance of getting home).