A friend pointed out to the home page of Subterranea Britannica (subbrit - not to be confused with an equally odd site britsub). SubBrit is a somewhat bizarre site documenting all the underground shelters, and other subterranean facilities spread over the UK. It seems surprising how many tunnels and underground bunkers there are around.
On the topic of concrete relics. I was given a DVD of the BBC series Coast. It's a very good programme,for example, one part that caught my attention was the segment on sound mirrors. These were some huge concrete reflectors made during World War 1 to allow a listener sat at the focal point to detect the sound of enemy planes flying over the channel - before the plane was visible. The programme demonstrates them working. However, RADAR was invented about the same time, and the ears fell into disuse.