Car number plates have a consistent, mandated font with a set of regular formats on high-contrast backgrounds that makes it almost trivially easy for computer vision to extract a number plate from an image of a car.
Cars are licenced transport. You are required to a) tax and maintain your vehicle b) identify who was driving the vehicle if you are the registered owner and requested to do so by the police and c) not obscure or obfuscate your number plate.
Faces are not licenced, are uniquely identifiable personal data - ignoring the enormous false positive detection rate from current technology - and everyone has one. Consider the implications for a supposedly free society (of which the United Kingdom no longer is one) where the state can draw arbitrary inferences from the presence of someone with a face that matches pattern X in location Y on date Z.