It does say in the highway code not to park in a marked bus stop, but the same rule also says not to park facing against the traffic flow, and how many people do this on a daily basis.
I'm always willing to give emergency or other official vehicles the benefit of the doubt, especially if you don't know the circumstances, and whether they're exempt from the normal rules. I've often seen people whinge about traffic wardens parked on double yellows.
In this case I don't know what the police were doing, whether the use of the van meant there was equipment or people that needed to be close to where they were working, or whether the road was chock full of cars and it would have meant parking the police van some distance away.
I'd rather see them in the van and on their way to the next job than wasting minutes walking back to the van if it had to be parked streets away.