On the subject of online resources, the Signalling Record Society lists among its publications a Signal Box Register for the GCR, listing 'basic physical and chronological details of every known signalbox.' It's only £4-20 plus p&p which can't be bad, given that I paid £3 for a pint of ordinary bitter the other day!
If you haven't got a copy of the 1914 Appendix they can also supply that for £12-30 plus p&p, which again is excellent value.
Finally, if you check out their list of available photos there are lots of GCR boxes among them. Oh, and there's even a list of signal box diagrams that includes some on the GC.