[sgtcallistan]

A classified 'above top secret' file containing the locations of a number of hidden bunkers in the British Isles.

Officially listed as the ROTOR system, part of ADGB (Air Defence Great Britain), built between 1945 and 1955 at great expense, and disused since 1962, they are now the remaining dispersed out-stations of the most secure location in the country (Brit Warehouse 23).

One, still marked on the map as RAF Fauld, is currently a crater about two miles across...

This above could act as an introduction / seed for a 'Brit. W23' campaign.

A little research can turn up real-world locations to use as part of this, paraleling the use of old airfields and army bases in the TV shows of the 1960's, as in the OP, such as the Avengers.
It would be fun if one could be found near Portmerion, Wales...