UK war time regional seats of government

An explanation caption from Wikipedia.
Regional Seats of Government or RSGs were the best known aspect of Britain's civil defence preparations against Nuclear War. In fact, however, naming conventions changed over the years as strategies in Whitehall changed.

In the aftermath of the nuclear attack on Hiroshima and the Russian acquisition of the atom bomb, it was clear that London could not survive a nuclear bombardment. Although considerable effort still went into secret construction of military citadels under London, the solution was to disperse the machinery of government into small pieces in the provinces, where there would be a greater chance of survival.

Experiments along these lines had taken place during the Second World War, when a system of Regional Commissioners existed and key departments were moved out of London to Bath, Harrogate and Cheltenham, among others. However, the idea of a Regional Commissioner dated back to the First World War and the 1926 General Strike.

It was then expected that Central Government might itself cease to exist, and control would pass entirely into the hands of a Regional Commissioner, of Cabinet rank, who would wield absolute power in his region. His staff would replicate all parts of Central Government. -From the Wikipedia .

Post atomic-war and post Nazi invasion function
Each of the Regional Commissioners would asses his facility's situation, send out people to survey the local area (and in a Cold War scenario- mostly die of radiation sickness), contact other command bunkers subordinate to his and prepare to run the region he control as best he could, by what ever means he could, in the hope that something could be salvaged from the post atomic-war disaster/partial nazi occupation.