We've all done it, you look in your mirror, see a car in the distance in the outside lane and make a judgement they are doing just over the speed limit so you'll be OK, nobody expects the car to be doing 120 mph.
This. Hence why emergency vehicles traffic well in excess of the speed limit tend to use these flashy blue lights so you are aware
To be honest, no one except OP's dad and the police driver know what happened.
Dad could have seen the car in the mirror obviously speeding and decided to be a vigilante and slow him down.
Dad could have seen the car a mile away behind, figured he had plenty of time to overtake (not expecting someone doing almost twice his speed) and pulled out perfectly reasonably and the police office took offence to being slowed down.
Dad could have not bothered to look properly and pulled out 20m ahead of police car, forcing him to slam on and perform an emergency stop.
etc.
If dad thinks he's in the right, take it as far as he can (be bothered).
If not, take it as a lesson to check your mirrors more carefully in future, count it as a lucky escape that he didn't end up with a car going into the back of you at 120mph, and drop the force in question an angry tweet suggesting they advise their drivers to use the blue lights when they're speeding down the motorway.
It reads like the second car came past after his dad had already been stopped.
True, I guess it depends how soon after...