I have actually seen a detector van! In the mid 90s parked ostentatiously between Bristol's Eye Hospital and the bus station miles from any residential area. Y'know, where lots and lots of people would see it. It looked like some pre WW2 experimental Bakelite radar contraption.
Anyway, if live streaming is the target of the Beeb's attention but catchup services are ok then a solution for the determined content thief springs to mind. How about a box that receives live streamed data and stores it for a minute or two, or a second or two, y'know just to smooth out the buffering, and then sends it on its way to the crim's screen. All technological mumbo jumbo aside what would the legal case against that person look like? If it's not live, if it's delayed by a certain amount of time then it's ok?