100 was a number.
A chalkboard at Coal Hill School in November 1963 referenced "100 Cents". (TV: "An Unearthly Child" [+]Part of An Unearthly Child, Anthony Coburn, adapted from The Pilot Episode (Anthony Coburn), Doctor Who season 1 (BBC tv, 1963).)
Sunday was 100 light-years from Earth, (PROSE: Wetworld [+]Mark Michalowski, BBC New Series Adventures (BBC Books, 2007).) as was Beta Caprisis. (PROSE: Sanctuary [+]David A. McIntee, Virgin New Adventures (Virgin Books, 1995).)
the Master once bribed an architect to make the Spikes of Doom, having travelled back 100 years before he was to meet with the Ninth Doctor and his companion Emma at Castle Tersurus. The Doctor travelled back to bribe the architect at an earlier point, causing them to instead be subjected to the Sofa of Reasonable Comfort. (TV: The Curse of Fatal Death [+]Steven Moffat, Doctor Who television episodes (BBC One, 1999).)
Captain Jack Harkness' stolen Chula warship gave a 100% probability of any attempt to jettison a Schlechter Wolf from the ship causing immediate detonation, as well as giving a 100% probability of Jack's death in under 2 minutes, as the bomb was going to detonate and there were no escape pods. Despite this, the Ninth Doctor saved Jack by materialising his TARDIS on the ship. (TV: The Doctor Dances [+]Steven Moffat, Doctor Who series 1 (BBC One, 2005).)
At Rose Tyler's request, the Ninth Doctor took the TARDIS a hundred years through the Time Vortex from 2005 to 2105. However, the Doctor claimed that it was "a bit boring" and so they went even further into the future before stepping out of the TARDIS. (TV: The End of the World [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005).)
A ship of 100 cryogenically frozen Veil’s were sent out into space but it crash on Earth in the 1970s. It got taken and store in a vault by the Alliance of Shades. With the help of Mister Dread, Androvax and the ship got into space where they go to find a new world to live on. (TV: The Vault of Secrets [+]Phil Ford, The Sarah Jane Adventures series 4 (CBBC, 2010).)
In maths, 258 minus 158 was 100. Rose Tyler answered this correctly in The Weakest Link. (TV: Bad Wolf [+]Russell T Davies, Doctor Who series 1 (BBC One, 2005).)
As Ood Sigma revealed to the Tenth Doctor when he returned to the Ood Sphere, one hundred years had passed following his involvement in the Revolution of the Ood. (TV: The End of Time [+]Russell T Davies, Doctor Who Christmas Special 2009 and New Year Special 2010 (BBC One, 2009-2010).)
The ISO used by Giles when he talked to a camera about PROBE and leads in a case regarding an upcoming perihelion was 100. (WC: Out of the Shadows [+]James Hornby, P.R.O.B.E. (Arcbeatle Press, 2021).)
Behind the scenes
References in invalid sources
Dave's choice of disguise when attempting to infiltrate the Doctor Who crew to find out spoilers for the new season was a TARDIS costume labelled "I am a TARDIS" and "100% real". (NOTVALID: The Daft Dimension)
The Doctor's Five Hundred Year Diary referenced "[sums] of human knowledge" of 25%, 50%, 75%, and 100%. When it displayed 100, the text was red. (NOTVALID: Tom Baker stars in John Lloyd's lost Doctor Who adventure, The Doomsday Contract)