Even on a good day the LHR T5 security work really slowly. They walk slowly, they move the security trays slowly, they scan the bags slowly in the bag scanner, they are just slow, slow, slow. They are all uniformly slow.
Pretty much every other airport I go through has staff moving faster: Stansted, Amsterdam, Chicago, San Francisco, Copenhagen, Dallas-Fort Worth, are only a few that come to mind from recent travel.
I don't know their thoughts or any other circumstances, but their behaviour seems to exactly match that of someone who is trying to do their job as slowly as possible, acting as if they want to do as little as possible in the time they are being paid to be at work.