It would depend on who attacks first. If Unthinkable happens because the Soviets backstab the Wallies that changes in a hurry.
That's not either Unthinkable variants though. Unthinkable variant 1 is the WAllies backstabbing the Soviets, unprovoked. Unthinkable variant 2 is purely a (post-demobilization) Britain vs the Soviets fight, with the Americans having retreated back into isolationism pre-war.