That, if Hitler cancels his plan to invade the USSR.
I personally don't think the war would escalate much beyond Iran, as the most the brits could do were bombing runs in the caucasus and a hard circumnavigation of either Norway or the Pacific to attack the huge place. Which would be useless as the soviets could just retreat to the interior and the British Empire had no land borders with the USSR. Would be a hard fought battle, as this was different from the Russian Civil War.