The problem with the scheme is that it stops lower income groups from owning and driving cars (some of whome may switch to illegally driving... but thats not my point), what needs to be done is to tempt people who don't normally drive use public transport i.e. wealthier commuters, people who can generally afford to keep driving when these schemes are introduced. Road fund tax should be much higher, and based more on the size of the engine, and the emmissions (rather like the current system, but with a higher granularity a separate category for 1.1,1.2,1.3 etc...) so as you approach the 4.0+ engines of more powerful cars. Perhaps take into account the income of the driver/owner of the vehicle so that relatavly everybody pays the same.
My point here is that car use should be reduced, but in order to do that public transport must be better: more convenient say by adding more bus routs (not just adding more busses); implementing/extending existing tram and rail networks; making stuff run on time more.
BUT: This can't be done over night, and its going to cost sooooo much to do. So the easiest way for the government to deal with the problem, is pick on motorists, basically what they're saying is this "we know public transport is crap, but there is nothing we can do about it, so all the money we're going to raise from taking the motorists is going to keep funding the existing system that we know doesn't work"