Re: It appears from the article that Mr. Wolmar is a railway enthusiast...
"who pays the road tax if no-one owns cars?"
For the most part, "who cares?"
Road tax (as in car registration VED) nets about £5 billion per year.
FUEL taxes (excise duty, various taxes and VAT) net something around £75-80 billion per year.
Maintaining the roading system costs £25-£35 billion/year depending who you listen to.
All that bluster about ringfencing VED for roads was misdirection. If that had been done the government could have kept its promise and _still_ reduced what it paid out in maintenance.
It also brings up another important point: Fuel taxes are a significant income for the exchequer. As soon as alternative fuels start harming that income the taxation will be transferred onto those alternatives somehow (either milage charges or mandatory fees per kWh used for transportation purpose).
Many years ago people who had CNG cars would invest in compression rigs that could "charge up" their car from the gas supply at home for about 1/4 the price of CNG on the forecourt. That was eventually curtailed. In a similar manner, if you're making your own biofuel there's a 2000 litre/year limit after which you must pay excise duty of 45p/litre on EVERY litre produced (not just the amount over 2000 litres).
e-fuels excise will likely be rated in the same way (2000 litre-equivalent exemption per household, then a large bill arrives)