This is going to take the long route. Continuity of Government.
"In the early hours of October 31, 1971, a bomb exploded on the 31st floor ""WHICH DEVICE?"" (curiously, no one ever claimed responsibility for hiding the device). No-one was injured in the blast but the physical damage took two years to repair."
"Why is it round? The architects that designed it wanted it to be able to survive a nuclear attack (so that the British government would still be able to communicate with the world, post bomb). They noted that the only buildings that survived in Hiroshima and Nagasaki were circular, allowing the enormous blast wave to surge round them, so they built it round."
If you believed that the Soviets had got it wrong and their bombs were dudds. After all that effort where would you stick a bomb to kill your civil control problem?