Trying to get signallers to operate any work station would be like trying to get train drivers to sign for all lines in mainland UK....
It's certainly possible to see a difference between a signaller who knows an area well compared to a less experienced signaller, who does not know the area so well. The experienced signaller will be able to maximise the use of the lines, routes, points etc. and know the length of time it takes trains to pass through a section. Whereas, a less experienced signaller will be far more conservative.
There are also practical technical problems with creating one super ROC. Yes, it's possible in theory. But in practice, the number of problems outweigh any possible advantages.
Plus, and this is an important point, Network Rail does not have, and the railway is never likely to have the money available to do this, even if it wanted to.
As it is, the different routes are using devolution to make their own decisions. ETCS is likely to be delayed further, as the money for this is coming from different sources, and some of these projects now have less money in CP7 than expected. So, less or no money for new expensive big ROCs.