Fun fact about Post Office Tower (aka BT Tower, Telecom Tower, and GPO Tower):
Its location was designated an official state secret as it was of considerable importance to the UK telecommunications network
This was despite the fact that it was a just under 200m tall building in the middle of London and was, at times, open to the public, with a restaurant at the top
Yeah, it wasn't until 1996 that anyone could legally tell you where it was
https://en.wikipedia.org/wiki/BT_Tower