This article contains spoilers for Doctor Who: Flux episode 4.
The trailer for Doctor Who: Flux episode 5 promises to fix one of Chris Chibnall's biggest mistakes as showrunner. Despite a long history within the show, the United Nations Intelligence Taskforce, better known as UNIT, was surprisingly written out of Doctor Who, in what some audiences regarded as a slight on the group's legacy. However, it now seems as though the narrative is backpedaling on that decision, reintegrating some popular Doctor Who characters as a result.
In 1968, the writers and producers of Doctor Who realized alien invasions were becoming almost everyday in the world of Doctor Who. They came up with the idea that humanity responded by forming a special body dedicated to protecting the Earth from alien threats, one run under the auspices of the United Nations; UNIT, the United Nations Intelligence Taskforce. It swiftly became an established part of Doctor Who lore, with Jon Pertwee's Third Doctor exiled to Earth in 1970's "Spearhead From Space" and becoming their scientific adviser, working closely with Brigadier Alistair Gordon Lethbridge-Stewart. UNIT has been a mainstay in the relaunched series as well, reintroduced during the first Russell T. Davies era and playing a prominent role during Steven Moffat's tenure as showrunner.
The removal of UNIT initially seemed to be one of Chibnall's political commentaries, both on the impact of the Conservative government's austerity policies - there was mention of a spending review that pulled funding - and on the decline of multilateralism, exemplified by Brexit. However, the trailer for Doctor Who: Flux episode 5 seemed to suggest that Chibnall himself is reversing on Doctor Who's UNIT mistake - revealing the return of Jemma Redgrave’s fan-favorite character, Kate Lethbridge-Stewart, who took over UNIT for a time partly in honor of her father. She's shown confronting the Grand Serpent, a corrupt official who's tied to Vinder's plotline; he mocks her for believing she is capable of protecting the Earth, which strongly suggests she's back in the driving seat of UNIT. The BBC subsequently released a promotional image confirming UNIT's return alongside Kate Lethbridge-Stewart.
Doctor Who: Flux episode 4 may well have already foreshadowed the return of UNIT, albeit in a rather subtle manner. The TARDIS deposited the time travelers in the village of Medderton in 1967 - on the night every inhabitant was destined to mysteriously disappear. This had happened once before, in 1901, and apparently it had all been orchestrated by the Weeping Angels; Medderton was subsequently converted into an army barracks. It is notable that, in Doctor Who continuity, the events that would lead to the formation of UNIT - the Great Intelligence's invasion of the London Underground - happened just a year after the second batch of disappearances at Medderton. It's therefore quite possible the army barracks was actually one of UNIT's headquarters. If so, it's rather amusing to note the Doctor even left the TARDIS there when she was spirited away by the Weeping Angels - and the conversation between the Grand Serpent and Kate Lethbridge-Stewart is in front of the TARDIS.
This would potentially mean UNIT has been guarding a TARDIS for a long time - although it's safe to assume they never mentioned as much to the Doctor. It's impossible to imagine Jon Pertwee's Third Doctor being able to resist the temptation of stealing his future TARDIS to escape his exile from Earth - at the very least he'd have studied the dematerialization circuit to figure out how to repair his own version. But now events have caught up with Doctor Who, and with UNIT too - meaning the current TARDIS is stuck on Earth, presumably sought after by the Grand Serpent, and with only Kate Lethbridge-Stewart and UNIT to protect it.