Who said the world tension is high enough for the democratic allies to take part in the war between the Germans and the SU? And why would they even want to intervene? The SU would love to conquer central europe as much as the German Reich wants to conquer the eastern part. The allies could let them battle it out and perhaps get rid of one of the 'evil' world powers.
Touche, if the world tension didn´t allow it, they can´t do nothing.
France and Germany are rivals since over 800 years, the UK ever wanted to keep the status quo, for their own gain naturally. Thats why both never ever let the germans become a juggernaut with approximately 200 mil. population and a gigantic industrybase.
In WW2 germany lost "only" 3-4 million soldiers on the east front, if they had won the war, they had lost fever soldiers. Germany drafted 17 million soldiers, so if germany conquered the Su, etc. and used the captured weapons to equip only 10 mill. soldiers, did you think France and Britain had a chance to stop them, from conquering France proper?
The same goes for the SU, if they conquer germany, poland, the baltic and maybe the balcan states.