Aside from the police, who else uses Automated Number Plate Readers? I saw an unmarked Toyota carrying the same cameras the police cars do.?

Only the police use mobile ones in the UK, but ANPR is available for use by other groups: The Dartford Tunnel, run by a private firm, has recently ditched its toll booths in favour of static ANPR and digital payments. I believe that's how the Congestion Charge in London is policed, too. The car park near me, run by the council, is getting ANPR installed so it doesn't have to send parking inspectors out to see how long vehicles have been there.

Both of these things introduce new technology at the expense of people's jobs, you'll notice. Such has always been the dilemma since the first cotton mills.

Unmarked Police cars in the UK.

could have been unmarked police car. only law enforcement uses them they don't sell them to the public and you would have to be tied into system that returns information on the plate, that is also not sold to public

Local council enforcement cars use ANPR cameras now as well.

private investigator might