There was a limit for approved CCTV transmission technology at the time but I don't think even Paignton would have exceeded it. Silk Mill near Taunton exceded all of these sites in disatnce from Exeter PSB, at just under thirty miles.
I think the decision to keep the local boxes was for other reasons including workload and the token issuing requirement in the case of Crediton. Paignton North was wastefully resignalled twice in the space of a few years. Firstly the North signalbox was abolished and control of signalling and crossing transferred to South signal box, remaining mechanical, with CCTV supervision. Then South signalbox was abolished and a new relay interlocking provided worked by the new small panel in the booking office. The original idea in Area Manager days was for the signaller job to be combined with that of booking office retail, but that never happened for various reasons and was soon superseded by privatisation anyway.
Exmouth Junction was a special case. It was intended to be a pilot site for a new VDU based precursor to IECC with relay interlockings, but the technology was not ready so a 'temporary' wooden NX panel was constructed instead with printed paper graphics beneath a perspex surface. I don't know if that is still there today, but it was in 2009:
https://www.flickr.com/photos/salisburyasc/6802586188/
The experimental VDU based system was manufactured but for reasons known only to those involved in the project was never considered suitable for installation. It remained as a demonstration sytem for IECC type operation in the S&T training school at Reading for many years, but I don't know of its final fate upon closure of that establishment.
Not quite. The hardware was and is a problem but the system at Exeter was more than a traditional TDM and presents additional challenges for certain parts of the control area. It was an experimental 'panel processor', where instead of hard wired relay logic the panel push button controls and indications were handled by a microprocessor based system requiring programming and data preparation. It was an early model of a particular series that was almost immediately discontinued by its manufacturer, and crucially its data configuration methods were no longer supported or offered. Hence following commissioning to the original layout requirement, no further software or site specific data upgrades could be carried out on areas of the panel that use the technology. The more local interlockings at Exeter St Davids itself, Cowley Bridge, Central and City Basin are not affected by this as the panel interface is conventionally hard wired using relay logic that can be altered if required by any suitably qualified signalling design house. These interlockings are remote controlled by large multi-pair cables rather than multiplexed transmission equipment. To the west, more distant interlockings at Dawlish Warren, Teignmouth, Newton Abbot and Totnes use the panel processor system with TDM transmission, as do Tiverton Junction, Silk Mill, Taunton and Cogload to the east. Improvement of the signalled connection to the West Somerset Railway to meet full passenger standards had to await the replacement of Silk Mill Level Crossing by the overbridge there today. This was because site specific data could not be recompiled for additional channels so channels previously used for the level crossing controls had to be reassigned and 'hacked' by external relay logic to become the new route setting controls and indications that were required...