P
From Tardis Wiki, the free Doctor Who reference
P (Morse code: .--.) (TV: The Man Who Never Was) was the sixteenth letter of the alphabet, following O. (AUDIO: A Death in the Family)
The Seventh Doctor kept a spare TARDIS key hidden in the cubbyhole above the letter "P" in "police box" as seen on the police box-shaped outer plasmic shell of the Doctor's TARDIS. (TV: Doctor Who, AUDIO: The Angel of Scutari)
Graffiti reading "TP8" in red was present in London on 25 December 2006. On the same day, a red crate labelled "LSC" featured white graffiti reading "LP". (TV: The Christmas Invasion)