British security/police seem to take their power a bit too far very often, in my experience. They have absolutely no problem with a little shouting. Of the first-world immigration officials I've had to interact with I'd rank them perhaps second worst, for general cluelessness and authoritarian behavior.
The typical British "don't say or do anything, just tut" behavior does apply often: Public displays of racism? Civil liberties being taken away? A simple tut will do. But if you're British immigration, then shouting incorrect or confusing instructions is just part of the job.
The number of times I've seen this is too high, and that's not even including the number of patronizing and wrong instructions I've gotten, either. For example, getting shouted at that I should have presented my American passport when I presented my Dutch passport (...what?)