UNIT HQ was the name given to the operating base of a UNIT branch and usually refers to the headquarters of the British arm. It is not to be confused with UNIT Central Control in Geneva (TV: The Invasion), the command and control facility for the entire international organisation.
List of known UNIT HQ locations
UNIT HQ has occupied a number of physical locations, presumably to help keep its location a secret or adapt to current needs.
Military plane
During the Cybermen invasion in the 20th century, UNIT HQ was on a military cargo-type propeller-driven aircraft. The facility included offices, a command and control centre, a vehicle bay and space and equipment for a UNIT Assault Platoon. The plane served as HQ for the officer commanding unit. There was also a Wing Commander aboard who held responsibility for flight operations. This mobile base allowed UNIT to move about more easily. (TV: The Invasion)
In the early 21st century, when the UK branch were briefly framed as criminals, they escaped the country in another military plane which also served as a temporary in-transit base. (AUDIO: The Wasting)
Building in London
During the Auton incident, UNIT HQ was in a building in London. This HQ included offices, a science lab and a parking garage. The guards did not wear UNIT uniforms, presumably to keep a low profile. (TV: Spearhead from Space)
There were two other HQs in London used after the first Nestene attack and prior to the move to Denham (see below.)
The HQ in the second Nestene attack and in use when Jo Grant first joined Unit, is difficult to identify and locate, as the images we have show interiors only. However, in footage of the incident when the bomb was thrown out of the lab window into the Thames, and other surviving views out of that window indicate that the building must have been on Lower Mall just East of Hammersmith Bridge.
Looking at buildings in that vicinity today, one can see an old (possibly 18th C.) house facing onto the river close to the bridge, and backing onto the City Arms Pub. (Probably number 3, Lower Mall) The first floor windows of this house are identical in shape and design to the one smashed in the Doctor's lab. Therefore it seems highly likely that this was an interim Unit HQ during this period.
The other location is 24 Cornwall Gardens, in Kensington, which was HQ during the International Peace Conference and again during the Axos crisis. There is some evidence to suggest that the Kensington Base was still in use at the time of the incidents with Azal. It seems likely that UNIT had two HQs at this time: one in Hammersmith and another in Kensington.
These HQs were temporary, and they later moved to Priory Mews in Denham. (PROSE: The Scales of Injustice, PROSE: The Face of the Enemy)
Building on the edge of London
The Third Doctor, after rescuing Doc Dantalion, requested that he sign over ownership of his 16th century manor house. This allowed UNIT to have a rent-free HQ, and its funding (which had been cut by the government) was restored. (PROSE: Where the Heart Is)
This HQ, on the edge of London, featured more visible guards, offices, a parking garage and a science lab. It was briefly transported to an alternate universe by the Time Lord Omega. (TV: The Three Doctors)
This HQ was visited by the Second Doctor in 1983 for a UNIT reunion. (TV: The Five Doctors)
Tower of London
During the Sycorax's attack of Earth, UNIT had a HQ located under the Tower of London. (TV: The Christmas Invasion) It was probably still used during the time of the Sontaran invasion of Earth, since Jo Nakashima was driving towards "UNIT Headquarters, Tower Bridge, London". (TV: The Sontaran Stratagem)
Later, under Kate Stewart's leadership, the Tower was used a base for tests on the mysterious cubes. (TV: The Power of Three)
Articulated truck
During the Sontaran incident in 2009, UNIT maintained a mobile base in an articulated lorry. The vehicle included offices and modern command and control facilities. Nevertheless, at this time, the official HQ was still in the Tower of London. (TV: The Sontaran Stratagem)