I personally cannot see the ANPR way working, I think they would be targeted. Fill them with expanding foam trick, or just a simple sticker on the camera lens. They would spend more fixing them, then getting money out of them.
I'll simply note down your registration number, go to Blackbushe market, have the plates made as "show plates" (no need for ID), put them on my vehicle, and then you foot the bill for me 
As you said earlier mate, an easily hacked system.
GPS trackers would be the way of choice I think 
Again, very easy to disable by simply blocking the signal (assuming I even present my car for the obligatory fitting in the first place).
All of which makes current system quite fair, more you drive, more you use in fuel. The more you pay!
I'd say we are being royally shafted under the current system with the amount of tax involved, but that's neither here nor there.
The proposed changes would mean that we pay "per mile", and that's a loser for anyone who does more than (say) 1000 miles per year (based on quotes provided by insurance companies who provide "by the mile" cover).
Don't think for one minute that if "pay per mile" comes in that road tax will be ditched, it ain't gonna happen mate....it will just be another tax that we will begrudginly accept whle being further beaten in to submission.
Compare it to smokers......
Last time I looked the government raked in £11 billion per annum in tobacco duty alone, now I'm sure you'll agree that isn't a sum to be sniffed at.
The government pumps out adverts about giving up smoking and all that crap, but what do you think would happen if all the smokers in the UK did what the adverts said and gave up?
Do you really believe the government will let £11 billion escape?
Oh no mate, every single last person will have their taxes (and instant fines) increased to make up for the shortfall.