UNIT HQ, City of London
By late 2023, the branch of UNIT that operated in the United Kingdom had its headquarters (TV: The Star Beast [+]Loading...{"timestamp":"00:49:15","1":"The Star Beast (TV story)"}) located in the City of London, near the Gherkin. (PROSE: The Time Traveller's Almanac [+]Loading...{"chaptname":"Everything Changes","page":"71","chaptnum":"3","1":"The Time Traveller's Almanac (reference book)"}) It had a helipad. (TV: The Star Beast [+]Loading...{"timestamp":"00:49:15","1":"The Star Beast (TV story)"})
History
The UNIT HQ was among London's skyline by the time the Meep's attempted Destruction of London was thwarted by the Fourteenth Doctor, Donna Noble, and her daughter Rose. (TV: The Star Beast [+]Loading...{"timestamp":"00:49:15","1":"The Star Beast (TV story)"})
Behind the scenes
This UNIT HQ, going off the on-screen geography seen in The Star Beast [+]Loading...["The Star Beast (TV story)"], appears to be located the financial district of the City of London, specifically the area surrounded by Fenchurch Avenue, Billiter Street, Lime Street, and Leadenhall Street, occupying the position of the Willis Building, the Scalpel, and the Moniker.
Therefore, the UNIT HQ is opposite the Leadenhall Building northwards, 40 Leadenhall Street to the east, the Garden at 120 to the south, and the Lloyd's building to the west.
Location information
Though it's possible that a real world location doesn't exist in the same geographic space in the Doctor Who universe, such cases are few and far between. Thus, the map to the right is probably a good indicator of the DWU location of UNIT HQ, City of London.