[jmkni]

There's a small caveat to this, technically in the UK the police are allowed to speed if they are heading to an emergency, they don't need livery on their cars or even sirens

So if you're in the outer lane and the person behind you wants to go faster than you and get past, get out of their way.

It's safer anyway, and also you don't know who they are or where they're going.

[hdgvhicv]

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).