Fun fact: The BT Tower, despite being 177m tall, was long designated an "Official Secret" and listing its address was illegal despite the fact there was long a restaurant open to the public at the top. In 1978 a journalist was charged for publishing the locations of a number of official secrets, and the BT Tower was one of them, but not named in the court proceedings - it was only referred to as "location 23" - because it was "secret"... For a secret, it's kinda hard to miss.