I seriously doubt the UK government has forgotten about VED or the reduction in fuel usage as EV'stake a growing share of the vehicle marke. I'm sure the Civil services departments have several alternative contingency plans that could be implements quite quickly by whichever political party has formed the government.
With advent of EV's with their inherent ability to take power from different places in ways that cannot be traced directly i.e Home charge, business charger, or public charger. I'm pretty certain will make pay by mile a bigger possibility in the near future.