T

T was twentieth letter in the alphabet, which, by most accounts, succeeded S and preceded U, (PROSE: The Dalek Dictionary, etc.) however, according to Sarah Jane Smith who recited the alphabet backwards while under mental attack by the Mandragora Helix, she placed V after T. (TV: The Masque of Mandragora)

In Teen Titans, the Teen Titans lived in a tower shaped like a "T". (PROSE: Father Time)

Graffiti reading "TP8" in red was present in London on 25 December 2006. (TV: The Christmas Invasion)

Luke Rattigan told Jo Nakashima that his name was correctly spelt with two Ts. (TV: The Sontaran Stratagem)

Behind the scenes
In the real world, T is immediately followed by U.