Why is a day-one conventional strike on Britain difficult?

Why would a conventional Soviet war not begin by attacking Britain directly on day one?

Without using ballistic missiles that could look nuclear, Soviet bombers would have to penetrate long, heavily defended approaches around Norway or through the North Sea and German-Danish airspace.

Britain was not connected to continental Europe by the Channel Tunnel in 1980, so reinforcement routes and ports would work differently. A conventional attacker could not casually use ballistic missiles without creating nuclear ambiguity. Bombers approaching from the north would have to skirt or cross Norwegian defences, while a southern route faced German and Danish air-defence systems. The geography makes an immediate attack possible only at substantial operational and political cost.