here we go again. "vehicle excise duty" is a tax levied on certain classes of vehicles for being allowed to use the road ergo it is a "road tax". To argue the specious point that it had its name changed in the 1832 Great Reform Act is losing the argument before you've started.
Let's try "how much extra tax should a cyclist pay for leaving his car at home?" "how much should my 4 year old daughter pay for her bike with stabilisers". "how much more tax than an electric car should a bike pay". "average bike does 100 miles a year and weighs 15kg" (probably far less as most langhuish unloved in sheds), "average car does 10000 miles and weighs 1500kg, so would 1/10,000 of the car tax be about fair?" and so on.