Against a sea of other cars you would never be able to pick out a single wireless signal like that.
What’s being talked about with the tracking /ANPR connection is called V2x communication. It’s already starting to be rolled out. The new Golf will have it, as will the ID.3.
It’s designed for cars to communicate with each other as well as roadside objects (traffic lights for example). It’s not inconceivable that it ANPR stations could poll every passing car, and if something doesn’t match then it’s flagged.
The main issue with using it as a way of tracking for tax purposes (at least in the near to mid future) is that most cars won’t have it for quite a while. Even if it became legislation next year it would be at least a decade before most cars (on the road) have it as standard. Retrofitting tens of millions of cars would be a pain in the ****.
If they relied just on GPS it would be flakey and super easy to fool for cheap tax. Cops would have to drive around with very expensive signal testing equipment to try and catch people 1 by 1.
There will always be people that flout the system, no matter what system it is. Most people don’t however. Some people use red diesel to avoid tax, some people clone plates etc etc. Rather than focusing on the small minority that will break the rules, why not focus on the best/easiest/most convenient way to tax people that aren’t breaking the rules.
Those breaking the rules can be dealt with by the police in similar ways today.