The first test cases will be fraught with subjective opinion... if a car is being followed by an unmarked police car in the middle lane before they turn on the flashing lights.... the car in front could legitimately claim that the car behind was following at the same speed, presumably at a safe distance[?], so they weren't causing an obstruction to traffic as the following car made no attempt to pass, especially if they are already doing 70mph. Mind you if people didn't stay in the outer lanes unnecessarily it wouldn't be a problem in the first place.
... and it should catch the persistant and malicious driving of those who drive in an aggravating and obstructive way to wind people up, especially those who deliberately slow down, while passing a slower vehicle in the inner lane, then speed up again to the legal limit staying in the outer lanes when clear of traffic then slow again approaching slower traffic... you know the ones. What a shock they will get when the car who is forced to constantly adjust their speed behind them suddenly turns on it's Police lights...