Alternate counterparts of St Paul's Cathedral existed in other realities. These locations shared similarities with St Paul's from the Doctor's universe, despite existing in parallel universes or alternate timelines.
Aborted timelines
The Trickster's World
In this reality, while the rest of London had become a blasted wasteland, St Paul's Cathedral appeared to still be standing, albeit partially destroyed. (TV: The Temptation of Sarah Jane Smith)
Alternate timelines
The Eye of the Giant
In a futuristic version of London in an alternate timeline, Liz Shaw was interrogated by nuns in what used to be St Paul's Cathedral. (PROSE: The Eye of the Giant)
The Glorious Dead
In an alternate timeline in which the Church of the Glorious Dead conquered all of Earth over its history, Sato Katsura allowed the ruined St Paul's Cathedral to remain as a warning to humanity against defying the Church's beliefs. Sato and Kroton briefly fought in and on the steps outside of the Cathedral during their battle. (COMIC: The Glorious Dead)
The Time Travellers
In an alternate 2006, St Paul's Cathedral was destroyed by South African terrorists. (PROSE: The Time Travellers)
Child of Time
In an alternate 2586, Emily Blandish and Honoré Lechasseur's investigation of a mysterious murder led them to St Paul's Cathedral, which was deserted and rumoured to be haunted. The Cathedral was destroyed in an explosion caused by the death of the Dæmon Mastho. (PROSE: Child of Time)
Parallel universes
Pete's World
St Paul's Cathedral existed in Pete's World's London, along with an adjacent London Underground station. (TV: Rise of the Cybermen)