You keep left unless overtaking. If an unmarked police car is behind you then that doesn’t matter, you still only sit in lane 2/3/4 when overtaking slower traff

You keep left unless overtaking. If an unmarked police car is behind you then that doesn’t matter, you still only sit in lane 2/3/4 when overtaking slower traffic in lanes to the left. There are a few exceptions of course - when lanes are diverging for example (so 12 going off one way and 34 going another).