It's a nice thought, but how the hell do you implement "pay per mile"
And don't say at MOT time, as that only works if you keep your car for a full 12 months after each MOT, plus you will just end up with a large bill every 12 months, on top of the MOT (and usually servicing costs)
Simple, add mileage to the V5C at handover or SORN of the car (if it isn't already), then adjust your tax code so it gets collected through PAYE through the year. Other methods of payment could be offered e.g. the "large bill" which we currently have to pay for our vehicle tax or instalments.
I'm sure you can come up with lots of "but what ifs" (so can I!), but the current system is full of failures to address situations: does the current car tax or petrol tax care which family member uses the car most or if some of the mileage is employment or pleasure or if the car is part of disability mobility.
The current system at least delivers some degree of "pay for use" via fuel taxes. The system is flawed because governments like to use multiple different taxes to "incentivise" changes in behaviour and to just add to the pot of general spending - We all know that taxes which are hypothetically for something like road maintenance or health insurance are just part of the chancellors income side of his balance sheet.
Headline income taxes are treated as sacrosanct and the stealth taxes keep being jacked-up.
Analysis may decide that just counting miles is unfair on some communities. An agricultural worker may drive many miles and I might spend 3 times as long just doing 5 miles across a big city, but in slow traffic, with lights and roundabouts and school crossings to maintain - Which of us should be paying more? The ever increasing camera-monitored roads, checking average speed or cameras enforcing urban congestion zones might be used to gather more money, but if you do more "road pricing" for key highways, motorways and bridges you encourage people to re-block the bypassed towns and lesser A-roads.
I suspect that we'll look back at fuel taxes and regret the passing of a simpler time!
The number of "stealth" taxes is set to rise and I don't have any faith in the current regime to use careful analysis to achieve the right solutions. Let's face it - What are the chances that Capita are awarded a contract to provide business analysts to look at the problem of dwindling fuel tax and that Capita decide to employ Capita to collect the new taxes.