ANPR cameras aren't common place and in most instances are temporary, there to capture registration plate for speeding offences through roadworks.
I think you are confusing ANPR cameras with SPEC cameras which record average speed over a set distance and are usually found at the site of road works.
ANPR cameras are more widespread than you think.
ANPR cameras which are used by the Police are not used for speeding offences.
Put it this way, every journey you make there is a good chance that it can be tracked via ANPR. (not every journey but most).