"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.