Andrew C!
Been around a while....
I remember following a police van doing exactly this a few years ago; I followed it for about 10 miles, losing it through the 30 limits and catching back up with it in the 60 limits.
Might have been a reason for that. I never had an issue with a motorist overtaking me if I was travelling well below the speed limit, provided it was safe to do so - even if I was in a T5, Impreza, Cosworth etc.
If you were in a car, and the police were in a non-car based van, you probably would catch up with it on the national speed limit roads. They’d have been limited to 50mph.
Police vehicles are not usually limited. Especially the marked ones.