I've stumbled across a document from an old magazine series by GEC Telecoms that covers Kent Coast phase 2 on
issue 31, 1962. It specifically covers Canterbury ECR's electro-mechanical SCADA system.
The article includes a lot of detailed information on how the system operated as well as a plan of the building's layout, multiple pictures of desks and equipment, and several circuit diagrams and other schematics
I've skimmed in and struggled to make heads or tails of it but still very interesting. From one I can understand, the control room comprised multiple distinct control systems each of which can operate a maximum of 4 substations with each substation having a maximum of 44 controls or indications. Canterbury opened with 24 substations, 22 track paralleling huts (TPH) and 4 grid connections (with the then "CEA").
Each system used 4 pairs of wire from the control room to the field: one for sending commands, one for receiving information, one for the substation alarm, and the final one for metering and synchronisation. TPHs and grid connection switching stations were considered 'satellites' of a nearby DC substation by the control system and so normally had to be contacted through the main DC substation . There was some ability for a control system to take over the substations of the next-door system should a failure arise. The operation was that the control room would transmit coded pulses of + or - voltages that would be interpreted as targeting a specific piece of equipment within one of the substations with an Open or Close instruction. The substation then responded back to confirm operation.
This meant the operation of the mimic board was quite interesting (or at least not what I expected) in this system. The ECO would change the setting of the key switches (that correspond to circuit breakers) to what was required, then pressing a Close or Open pushbutton that corresponded to that Control System which would update the substations with the position of the switches of the mimic board. Or indeed the opposite, where a substation would contact the control room to say that a breaker had changed state (by way of illuminated lamps and alarm bells) and the ECO would have to change the switches to match what had happened.
There is also an article on the South London installations in the mid 1950s (at Lewisham, Raynes Park and Selhurst control rooms) in
issue 23 1957. The London control rooms are interesting because they were part of a total traction power modernisation. Previously the area was powered by 11kV distribution circuits from the railway's power station at Durnsford Road (near Wimbledon) and from the CEA's power station at Deptford road all at the non-standard 25Hz. This was to be upgraded to 33kV from CEA sources and all at the standard 50Hz.