Which is something you pay, at least in abstract, for someone else to clean up the carbon dioxide you litter the atmosphere with. If you dpn't want to pay the cleaning fee, don't litter -- take the bike!
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