From TARDIS Index File, the free Doctor Who reference.

The Post Office Tower, later known as the BT Tower, was a large tower in London designed to carry telecommunications traffic to the rest of England.

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It officially opened in 1966. That year, the self-aware computer WOTAN occupied the top of the Tower and directed preparations for C-Day, its robot War Machines and mind controlled human slaves from there. (DW: The War Machines) Later in that year, with WOTAN defeated, Igrix the Kustollon had left his spacecraft there. (DWM: The Love Invasion)

As the Doctor raced to get to the Igrix's ship, unknown to both parties, he passed his former companions Ben and Polly, both visiting the Tower. Both companions had had involvement in the C-Day affair. (DWM: The Love Invasion)

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Post Office Tower