That's why I said I agree with it, the theory would be OK. No fuel tax, no road tax, just pay per mile, heavy road users pay a lot and people that don't drive very often don't pay as much.
The problem is that we all know full well the price 'per mile' will be ludicrously expensive, even if you're on a deserted road in the middle of the night you'll likely be paying 30p a mile, and we know that fuel prices won't drop to anywhere near the level they should be (they'll drop to 75p 'to start with' and everyone'll say "Ooh that's a good start" and then they won't go down any more!!).
Also I was basing it on the fact that they WON'T measure your speed, and I'd expect to have to sign something that says they won't register your speed in any way.
Otherwise, just remove all road tax, speed cameras and everything, and increase fuel tax, that'll do the same job won't it?