Its wooden. All the ones still in situ in edinburgh are cast iron. There were originally 70 or so police boxes in the city. Only 50 or so are still around and accounted for meaning 20 have vanished. My one suggests that a bunch of those missing ones were probably wooden and perished.
Mines the same in almost every detail as the iron ones – in fact its more detailed. The wooden construction makes sense of the design too, so it seems the iron ones are facsimiles of an earlier wooden design.
Mine must have been taken out of service pretty early on. The ones you still see around edinburgh have what looks like a little chimney on the top – thats the 4 Minute Warning – being an existing set of networked locations with power and phone connection they were commandeered for civil defence duties and fitted with sirens during the cold war. Mine doesn’t show signs of that having been fitted so it was presumably taken off the street before then.