There is a world of difference between the 'book' and the reality and immediacy of a real life situation such as this. In reality, officers regularly indicate to drivers the proximity of where to stop and in reality they wouldn't take kindly to someone not doing so whether they acknowledge their presence or not.
While some know-it-all perfectionists might like to think I'm talking out of my backside, and while I'm not and never have been a traffic officer, I do have some close connections

In any event and yet again, you only have to watch one of the many traffic cop programmes to see the reality of this over and over again.
As I said to the OP, and on topic, I firmly believe he cannot be subsequently prosecuted by anyone for stopping under the instructions of the police.
Regards,