"We deal with the odd, the unexplained, anything on Earth... or even beyond"
UNIT is a strange organisation, are they an elite band specialising in extra-terrestrial warfare, or a more conventional rather tiny organisation which is worryingly ineffective? It depends on the story. Also there's the question of UNIT's organisational structure which is a bit inconsistent and from which most people can likely only name a handful of obvious names. To the casual observer they are led by a Brigadier General, who under him has a single Captain and Sergeant, and the occasional Corporal. Yet there there is a raft of others of similar rank that simply appear at random and play no further role in the series. In some episodes, the organisation is even short on basic troops meaning that senior officers have to lead the assault on the aliens. I'll base most of this on the original TV stories, as there is more written in the books expanding the total number of characters further, but the number of one-off characters I'll describe later gives plenty to discuss.
UNIT was established somewhere between the Web of Fear and the Invasion. In the Invasion they have significant resources though a large number of troops, vehicles, heavy weapons and the ability to call in air support which they do in later stories like Terror of the Autons and Seeds of Doom. This was UNIT at its most powerful. What we see in later stories is an apparent decline in UN funding as they appear to struggle to pull together any resources at all. In Spearhead from Space they have large numbers of troops and are handling a clean up operation to collect the fallen meteors. In fact they turn out significant troops in most stories of Season 7 and handle large operations in Ambassadors of Death and the Silurians. Also of note is that in Spearhead from Space the Brigadier notes that "Since UNIT was formed, there’ve been two attempts to invade this planet", which, as Web of Fear led to the formation of UNIT, would strongly imply that there is an unseen invasion that they successfully handled without the Doctor's assistance!
But from Season 8 onwards it all started to go a bit wrong. Noting the quote at the top of this page, their remit appears to be one akin to the X-files, yet what we see in actuality are regular pitched battles aliens interspersed with mundane tasks such as providing security various political conferences. Maybe this indicates that UNIT are not taken seriously by either the British Government or the United Nations. Regardless of the numerous alien invasions foiled and lives saved, they have their funding cut and and are taken away from the main duties to provide security for things like World Conferences (Mind of Evil) that really don't appear within their remit.
UNIT has its authority undermined though other means as well. In Claws of Axos this directly threatens the world, as UN troops are arrested and confined by regular British Army troops while Axos carry out their plans. Again, in Invasion of the Dinosaurs they find themselves under the heel of the regular army again. One wonders if the Brigadier finds himself increasingly brought into the politics of funding that requires he comply with the British establishment even where this compromises his own organisation's effectiveness. This also explains his increasing time spent in Geneva and away from UK operations in Android Invasion and Seeds of Doom.
The issue of funding might also be reflected in the way UNIT recruits replacements and makes promotions. Promotion seems pretty rare in UNIT, can't they afford the wages? And it's not just because life expectancy is short. Lethbridge-Stewart is promoted to Brigadier apparently in order to take charge of the British branch of UNIT, but remains a Brigadier into retirement. Corporal Benton becomes a Sergeant early on, and, even though he's a bloody hero though many stories, only gets a further promotion to Warrant Officer after Mike Yates is discharged. Mike Yates is the third prominent member of UNIT as a Captain. Mike Yates appears a decent soldier but is brainwashed in The Green Death. This actually makes a good story arc into Invasion of the Dinosaurs, as he's supposedly had a period of leave but falls in with the Golden Age lot. This doesn't really seem much like the Mike Yates of old, and I like to think that his experiences in The Green Death weakened his mind and made him very vulnerable to suggestion. He is, after all, discharged and not imprisoned.
As mentioned earlier, to the casual observer it would appear these three, along with the Doctor and a number of disposable squaddies entirely comprise the British branch of UNIT. But fortunately there's a large number of other officers including Majors, Captains, Sergeants and Corporals that all make one-off appearances. This only gives a glimpse that the organisation is bigger than regularly seen onscreen. For example, in the Three Doctors UNIT HQ is directly attacked and only a modest amount of troops are available to defend it. Is this all they can draw upon? Or are we to assume that they have troops positioned around the UK? The other possibility that generously comes to mind is that due to the large experience UK UNIT troops have, due to the frequency of alien invasions targeting the home counties, they have been posted around the world to train and advise other branches. This certainly seems suggested by the more international feel to UNIT in Battlefield. While this might explain the high turnover in higher ranks, and why various captains and Sergeants make single but non-fatal appearances, it would explain the general shortage of troops in the regions most afflicted by alien invasion.
The relationship between regular army and UNIT also appears conflicted. We can see that on occasion it is particularly strained, but regular forces are drafted in to assist UNIT. This is quite a blurring of lines, as UNIT's work is supposed to remain secret. How do they stop the regular army discussing what they have seen? Possibly these issues are the reason Major General Scobie is tasked with liaising between UNIT and the regular army in Spearhead from Space. How do UNIT recruit anyway? If they are an alien-fighting elite, then their selection process may be like that of the SAS. But on the face of most evidence, given their combat prowess, it probably isn't. And how do you recruit people to fight aliens without telling them about aliens? Perhaps this is a clue as to the reason why there's a high turnover of staff within UNIT; soldiers from the British Army do a short stint with UNIT before returning to the regular forces. In Scales of Injustice Captain Munro is suggested to have returned to the regular army after the events of Spearhead from space. This seems quite plausible but how they keep a lid on all the secret warfare they carry out is anyone's guess. Although, as is the case with most alien invasions in the Doctor Who universe, the activities of UNIT are an open secret while the public simply refuse to believe all evidence proving the existence of alien attacks.
Lastly it would be worth mentioning the spoof UNIT recruitment film produced in 1993. The only recruitment film in history to show soldiers dying... frequently. "Our turnover is a bit high..." it admits and testimony from one soldier "I've lost a lot of good mates in action" is hardly going to whip up much enthusiasm. Is the hazard pay particularly high? In part 2 I'll discuss all those many soldiers that have fought with UNIT, their many Captains and Sergeants, and take a guess at some sort of command structure.
Until then, raise a glass to the many unnamed squaddies who have died fighting to save planet Earth.