There is an article in the February 2015 (issue 92) of the Hornby Magazine on "Signalboxes", written by Evan Green-Hughes. This includes exterior box views of Kings Dock (Swansea) in 1978, Gara Bridge in 1960, Edinburgh Waverley in 1966 and Billingham in 1965. There are interior views of Newcastle PSB in 1961 and Stockport No. 1 in 1968.
The article goes into some detail about the spacing of early signal boxes, although he does not mention the restrictions imposed by the BoT on operating points mechanically which influenced the siting of many boxes. The article goes on to explain briefly NR's intention of concentrating signalling into 14 centres (sic).