I encountered a brake tester and a bunch of useless police on the M25 today. There was a Fiesta hogging the outside lane and an E46 sitting a good distance behind. Eventually, the E46 got tired of everyone undertaking him and the Fiesta, and flashed his lights to get the Fiesta to move over. The Fiesta responded by brake testing him very hard, to the extent that the car squirmed under braking. There was a police van behind me who saw the entire incident, and rather than pulling the Fiesta, they just undertook both of them and drove away into the distance. I followed them for a bit and they did two terrible overtakes, causing other cars to brake, and then drifted completely out of the lane for no apparent reason.
I'm far from a police hater but I found that pretty shocking - if I hadn't seen it with my own eyes I don't think I'd have believed it.