The portions of this episode that take place in 1967 are the earliest chronological appearances of Lethbridge-Stewart (mentioned by name, and his voice his overheard through archive recordings) and UNIT. Colonel Lethbridge-Stewart (Nicholas Courtney) first appeared in The Web of Fear: Episode 3 (1968), and when he returned in The Invasion: Episode Two (1968), he had been promoted to Brigadier General and commanding officer of the newly-formed UNIT. Both were regularly featured in the Third Doctor era, 1970-74, and recurring periodically through the rest of the classic series run. Lethbridge-Stewart made a final appearance in Enemy of the Bane: Part One (2008) before Courtney's death in 2011. He was mentioned several times in the revived series of Doctor Who, and his daughter Kate Stewart (Jemma Redgrave) has become the new leader of UNIT.
Millington's line about "the whole thing at the Post Office Tower" is an allusion to the First Doctor serial The War Machines: Episode 1 (1966).
An announcement on the UNIT PA requests the attention of "Lethbridge-Stewart", and General Farquhar mentions that the voice was the new Corporal. Lethbridge-Stewart refers to Colonel (later Brigadier-General) Alastair Gordon Lethbridge Stewart (aka "The Brigadier"), and the Corporal is likely Cpl (later Sergeant) John Benton, two of the core characters from the UNIT era of Doctor Who.