Geneva

Geneva was a city in Switzerland where the international headquarters for UNIT and for the International Space Command were located (TV: The Tenth Planet). During the Brigadier's tenure with UNIT he consulted frequently by phone with Geneva. (TV: Spearhead from Space) The CIA's files on the Torchwood Institute had referrals going back to Geneva and above. (TV: The New World)

The location of UNIT's central headquarters in Geneva presumably has to do with the number of United Nations organisations centred there.