Now you don’t…
This stamp of the Post Office Tower from 1965 is superb, even if it misses out the Post Office Tower itself due to a printing error (thanks to @stampmagazine for the image).
In fact, that seems kind of appropriate as the Post Office Tower was deliberately left off Ordnance Survey maps for decades because it was deemed to be an official secret and therefore of such great military importance nobody was allowed to know where it was even though it had become one of the most recognisable buildings in Britain pretty much as soon as it was opened by Tony Benn (who narrates this brief history of the tower).
It even appeared in an episode of Dr Who in 1966.
The Post Office Tower was bombed in 1971 (often attributed to Irish nationalists but more likely the work of the Angry Brigade) and even survived an attack by a giant kitten in the 1970s.
It’s still very popular. Here’s a film by somebody who collects memorabilia about the tower.