Location 23

When erected in 1965 The Post Office Tower, as it was then called, was Britain's tallest building.
But it was classified as an Official Secret, as it was considered so important to the country's telecommunications infrastructure.
It was referred to as 'Location 23'.
So under the Official Secrets Act it was technically illegal to take photos of it, and it wasn't included on any Ordnance Survey maps until the mid 1990s!

[always amazing what you can pick up watching an episode of 'QI']