Forgive me if this is wrong, but I think the arrival refers to an arriving passenger waiting to go through border control. I have rarely seen departure security queues worse than Heathrow or Gatwick at stansted since about 2006.
The border control is staffed by GM Gov border force not MAG. It's true there are often lanes not open and not all automatic scanners working.
Is it mag who say how many border force agents there should be there or are they just told this is what you get?