The whole point of this taxation change is to account for ongoing electrification. The government are concerned about lost tax revenue from fuel duties and need a new mechanism to fund public roads through taxation by those who use it the most.

It's not just about pollutants.

That's a second-order concern - the view that fuel taxes are effectively road usage charges is already cemented by the fact that "red diesel" (fuel with an added chemical colorant) used in agriculture has a much reduced taxation regime vs regular pump diesel.

Tractors do use the road and as large and heavy vehicles I imagine they do a disproportionate amount of damage. I think tractors are just considered more essential so low fuel tax can be considered a subsidy of sorts.