What is the function of UK Border Force agents at UK airports between departures security and the shops?

First things first. We do not pay the Border Force to amble aimlessly about in airports, nor to cluster at departure walkways without a specific purpose. While it’s true that sometimes a preventative show of force may be called for, this is done by the police. Stansted has a dedicated police force to accomplish this (as does Heathrow, as does Gatwick) and they concentrate at the outside airport drop-off point in the first instance, but since you were inside at the time you wouldn’t have seen it anyway. The police will also concentrate at tube stations serving the airport.

Next, Stansted is hopelessly understaffed and there are few, if any, resources available for exit checking. Exit checking is done by interfaces to the various airline systems and dovetailing the information they get to their own systems. It means exit checking is primarily a “back office function”. Even worse, more often than not, the primary control point for arrivals is staffed by HMRC officers who have little or no clue how the rules work.

So why are they in position where you saw them? Let’s look at some of the possibilities…

• Stansted is a site where forced removals take place (as is Heathrow,
  as is Gatwick). These occur when the Home Office has chartered a
  flight to some place like Pakistan or The Gambia and seeks to return
  the most recent group of illegals, usually more than 50, and often more
  like 80+. When there’s a forced removal scheduled, the Border Force
  will concentrate units at various “choke points” throughout the
  airport (including the arrival of armed police units outside the
  airport). While the removal cases themselves are handcuffed and channelled through
  secured passageways not accessible to the public, they know that
  trouble can occur anywhere during these events and so they have to be
  ready to swarm. There is nothing to prevent them from challenging
  passengers walking through the public channels and they are entitled
  to do that, especially if the person looks suspicions. But
  essentially they are waiting to swarm if the situation calls for it. When the removal is over, they will stand down and head for the locker room or cafeteria or tarmac inspections of freight and animals or even back to bed.
• Next, the chances are REALLY good that if you see a lot of them
  clustered in an odd place, away from the primary control point, they
  have received a tip-off and are waiting to swarm on an arriving
  or departing passenger. They would be beyond the security checkpoint
  because they may have to run the guy’s photo and this could take a
  while. Security may want to stop the flow of people through the passageway until the arrest has been made and the “choke point” is great for accomplishing this, or they may need to catch the person’s accomplices along with
  the person, or watch what the person does before arresting them, or
  whatever. You’ll notice an exit to the side nearby because they like to minimise an enforcement exposure to the public eye (it gets people upset) and the passageway you describes sounds perfect for squirreling the person out of sight quickly. There will also be plain clothed agents in the shops area. Tip-offs are the major source of how the Border Force’s
  intelligence arm works.
• And finally it could be just any crap training lecture they are
  receiving on some finer point of immigration law or customs law. For
  all you know those agents may be Brazilian or American or HMRC agents or graduate trainees
  or anything else.

It’s within your rights to ask them outright what they are doing there; and it’s within their rights to tell you to take a hike. Personally I don’t recommend it, it can distract them from their mission.

Further reading: An Inspection of Border Force Operations at Stansted Airport

Also: An Inspection of the Intelligence Functions of Border Force and Immigration Enforcement

“What do these Border Force agents actually do, if not conducting exit border checks?”

The missions of these various agencies are often very broad and overlapping.

It’s quite possible they were there just as a ‘show of force’, with no specific tasks. Agencies will often take turns doing this, Border Force one day, Airport Police then next, HMRC after that.

The simplest answer is that those Officers were there because that was their post for the day.

For clarity, I am familiar with these situations. You can always ask the Officers, politely. But keep in mind, they probably cannot tell you anything about their specific task, if they had one.

To be perfectly clear, yes, it can be that simple. They are there just to be there, with no other specific mission.