There are multiple externalities to consuming petrol/driving cars around the UK.
There's a local/national one. By consuming petrol you're generally inflicting wear and tear on existing roads and adding to the need for road-space - requiring the construction of new roads. This is the externality that UK petrol tax is supposed to pay for - road infrastructure maintenance and construction. Ok it gets all mixed up in the general UK budget but, in theory at least, the tax pays for this externality.
But the government doesn't collect petrol tax and hand the money over to distant foreign countries in order to pay for global environmental externalities. So this externality is not paid for by UK petrol consumers?