The BBC has an interesting story  on the UK's Burlington facility, built as a cold war bunker in 1957:
Of particular interest to telecom historians is the large plugboard system, which by size may well be one of the largest intact installations left in the world. The BBC article claims that it was "the second largest telephone exchange in Britain". The system is built of good old-fashioned analog electronics, to hopefully continue functioning after an electromagnetic pulse.
The photos of the communications area show the facility in detail , as well as a 360 degree panoramic view  (requires Java browser plug- in).
The Guardian also has an article with considerably more detail  which notes there were 54 telephone operator positions.
Since the facility has been decommissioned, hopefully the plugboard can find a home in an appropriate science or telecom museum.