It works because the Postman driving the vehicle is not repeat NOT permitted to open his van for anyone. So the police, if they wish to stop him, have to follow him to the nearest Crown Post Office (NOT your local sub P.O.)where a Royal Mail Official will take charge of the vehicle and contents and then the police may speak to the driver (breath test him/her, arrest him/her etc).
In my days with the Royal Mail we had a card with that message printed on it to show, through the locked window, to any policeman who tried to stop us.
Some of these signs can now, I understand, be activated to light up thus requesting the police to follow the van to the nearest Crown Post Office. This could happen, for example, if the driver was under duress (someone was pointing a gun at him etc) or if he had been requested to stop by the police.
Even for a policeman to impede or wilfully delay Her Majesty's Mail is an offence.
That is how it works.
And in answer to your last question ~ Yes I have seen a PO van being followed and indeed have been driving one that was followed.