Distance between signals.
There is no set distance as such. It all depends upon the (lengh of section working), and that in turn depends upon the density of traffic movement, the proximity of junctions, sidings, platforms, stations, motive power depot's etc..
All these factors have changed over the years since the railways have been downsized a lot, and since the introduction of automated signalling and the terminating of guards on goods trains. Why even the signal boxes are now built without windows, that's if they are still called signal boxes?
Ken. ( ex B.R. signalman)