Singapore

The Republic of Singapore was a city/state in southeastern Asia on Earth during the 18th - 21st centuries.

Charley Pollard was on her way there when she hitched a ride on the R101, to meet up with Alex Grayle at the Raffles Hotel on New Year's Eve 1930. (BFA: Storm Warning)

The Doctor eventually got her there for her rendezvous, while the Doctor encountered Alex's grandfather Sebastian Grayle, who had subtly set up the rendezvous to gloat over having killed the Doctor in a previous era. (BFA: Seasons of Fear)

Major General Scobie, as a British Army officer, was posted to Singapore in the 1950s. (MA: The Scales of Injustice)

Behind the scenes
Singapore was supposed to be the primary location for the aborted Doctor Who story Yellow Fever and How to Cure It.