What, on British TV, is most likely to make you drop the remote and watch while surfing channels?

For me it would have to be Border Security: Australia's Front Line. I'm addicted to seeing what old South East Asian women are trying to get into the country. Also the guys working there all seem like pretty nice people. No glee in catching people just professional and firm. Can be heartbreaking also.

Of actual British TV Motorway Cops will always make me pause. Morbid curiosity overcomes me.