In the 1970s, the British branch of UNIT originally selected Liz Shaw to be their scientific advisor, but this role soon went to the Third Doctor, with Liz staying on as his assistant. The Doctor served as UNIT's scientific advisor throughout his exile on Earth. (TV: Spearhead from Space onwards) Although the Doctor ceased to stay on Earth on a regular basis after he regenerated into his fourth incarnation, his affiliation with UNIT remained, with others such as Colonel Mace noting that the Doctor had never officially resigned his position. (TV: Robot, The Sontaran Stratagem) Even when not actively working with UNIT, the Doctor wasn't above using his UNIT credentials to establish authority to investigate such matters as disappearing Concordes (TV: Time-Flight). Even in 2001, the Doctor retained a legendary reputation among UNIT, with the Brigadier musing that a three-second silence after learning that the Doctor was present during a crisis was an impressive incident (PROSE: The Shadow in the Glass).
Other occurrences Edit
While investigating the strange abilities of apparent medium James Lees for Queen Victoria, the Fifth Doctor was officially appointed Queen Victoria's scientific advisor, being offered to remain in the role even after his departure (PROSE: Empire of Death).