The police never stated that they passed Jeremy's village when travelling from Witham to WHF. They probably joined Jeremy's route shortly before they overtook his car.
The figures in the images linked above are unreliable. For example, it is stated that Pc Myall drove for a distance of exactly 1 mile at a speed varying between 25 and 30 mph. That would imply that the journey time was between 2 minutes (corresponding to 30 mph) and 2 minutes 24 seconds (corresponding to 25 mph), but the journey time is given as 2 minutes 45 seconds. It's then stated that this journey time corresponds to an average speed of 27 1/2 mph, but it doesn't. It corresponds to an average speed of 21.8 mph. There's no justification for the subsequent conclusion that Jeremy's car was travelling at no more than 20 mph when EP passed it. That conclusion can't be obtained from the figures given.
I don't understand your reference to Jeremy putting on a jumper. What evidence is there that Jeremy put on a jumper while making his way to WHF? If he stopped to put on a jumper after his car was overtaken, the time he took to do that would need to be estimated and subtracted from his estimated journey time (for the final mile of his trip) to allow his average driving speed to be estimated, and that estimated speed would then be higher than it would be if Jeremy hadn't made such a stop.