This is not exactly news to most people, but Road Tax, or Road Fund Licence are not exactly appropriate terms for the little round disc on your screen. Vehicle Excise Licence, which shows you've paid your Vehicle Excise Duty are better terms.
If anyone on here has attempted to tell a cyclist that "I pay my road tax....." they are highly inaccurate. It was abolished by none other than Winston Churchill and subsequently became a general taxation.
It's always struck me as being a little odd that the Congestion Charge should be linked to emissions. A Prius causes no less congestion than a Mitsubishi Evo, or one of the new Mercedes A-Class AMG. It does, though, arguably, put out less emissions.