...in the 1970s
According to the BBC (and a number of other media outlets), documents marked "Top Secret, UK eyes only" were recently de-classified by the National Archives, and said documents show that the Labour government of 30 years ago was engulfed in a furious row over the inadequacy of the nation's defenses.
In the air, the UK would be forced to confront an estimated 200 Soviet bombers with 98 fighters, resulting in destruction to the UK many times worse than that delivered by the German Luftwaffe.
So why didn't the Soviets attack? Well, probably they didn't realize the extent of their military superiority. Also, um...alliances? Pretty sure, the US and few European countries would have gotten involved, although maybe in the wake of the disaster that was Vietnam, who knows what countries would have been willing to do. Missed opportunity or missed debacle? Your call.