Is that same level of tax levied on heating oil? If so, that bolsters the case that it's a carbon tax. If, as in the US, heating oil is untaxed or is taxed at a fractional rate of automotive fuels, that suggests that it's a road tax and not a carbon tax.

This chart suggests that it's the latter (substantially a road tax more than a carbon tax): https://www.gov.uk/guidance/fuel-duty#rates