The story is a little more complicated. Thank you for the mention of 'A Vertical Empire' [I speak as the author!].
We also run an annual conference, and one of our regular attendees is a chap from English Heritage, very much into Cold War archaeology [he is the co-author of a very impressive book on the Cold War sites in the UK]. He discovered the remains of what appears to be a Blue Streak silo, but it would not have been an operational site. To save money, this, like its counterpart at Woomera, would be constructed in the side of a hill - in other words, you wouldn't have to dig a hole in the ground. He has published his findings in the third issue of our journal, Prospero [plug!].
One of the purposes of Spadeadam was to static test the missiles. It is likely that the silo there would have been used for training purposes - in other words, it would be fitted out as if it were operational, but not actually used as such.
By the time of cancellation, no sites had been selected, despite some years of investigation. The water table was a problem at Duxford, which was the first choice for building a silo.
I came across a couple of files in the PRO which I really ought to put onto the site: these were the designs for the silo lid. Two firms drew up designs: the lid was to be 400 tons weight and move back on rails. Although the site was supposed to be proof against a one megaton blast half a mile away, I can't help wondering whether the rails might have buckled in the blast, thus jamming the lid, and rendering the silo useless!
Nicholas Hill