Motor tax is based on the construction and use of a vehicle. Motor tax has been charged on the basis of carbon dioxide emissions for private vehicles (which comprise 78% of the vehicle fleet) since 2008. There are no plans to move from that environmental rationale towards taxing vehicles on the basis of width.
Applying width as the basis of motor tax for other classes of vehicle could potentially increase motor tax rates for vehicles that are larger by virtue of their function, but which provide an essential public or economic service, such as buses and heavy goods vehicles.