It's also a dead-end tax. Electric cars are coming, whether people are prepared for them or not. Increases to fuel taxes would result in an even faster shift away from internal combustion vehicles, meaning the tax base shrinks even more.
The government should focus on coming up with a tax which will work in a world of electric cars. Until now, we've been able to use fuel taxation as a proxy for vehicle use (bigger vehicles driven more will use more fuel, so more tax is paid). Since electric cars will be charged in domestic settings and possibly even using locally-sourced renewable power, there's no longer any centralised point to apply fuel taxes. The only way to get around this and still charge by kWh used would be for cars to report their energy usage. This is obviously entirely technically possible, but once you're at the the point of cars performing mandatory reporting of energy consumption, you may as well just make them report all their movements for pay-by-use road charging too.
In a world where rubber tyred vehicles move over to electric propulsion, the damage they do will reduce down to their land use and safety. A 2.5 ton Tesla used to do a school run in a dense area of London is doing a lot more damage than a 1 ton petrol car used out in the countryside, or a rattly old 3 litre diesel Land Rover up in the Highlands and Islands.
The ideal tax system for vehicles would mean all journeys are pay-per-use and vehicle tax ratings would be based on factors like mass and size, or whether they're equipped with extra safety features like pedestrian collision avoidance. It would be best for any such tax to be applied universally across all vehicles, even commercial and public transport ones, as the point would be to reveal the costs of driving. We need a tax system which still encourages bus companies to buy lighter and safer buses - the extra tax cost can be made up with increased subsidy in the short term.