If the police car was doing a 'vascar' check then yes, they follow, etc.
They could have been hiding behind you so the vehicle in front of you didn't see them? Especially so if a van.
The police car may well have had vascar on board, but they almost certainly pull you over if they want you. If you slow down as soon as you see them that seems to be enough to get ignored usually.
The police car was probably part of the ANPR team and I they presumably communicate via radio. So he might not have known who to go for or was waiting for a better description, or just wanted to get a missed numberplate or... anything really.