"when the roads are mostly paid for by the cost of the London Underground"
Taxes on fuel used in motor vehicles (duties and VAT) are something like 30bn GBP/year in the UK.
London has about 1/6th of the UK's population, so let's assume 5bn/year comes from London.
That's 5x the annual operating profit of London Underground[0].
Government income and expenditure is fungible, so it's hard to determine precisely whether X is paid for by Y or by Z.
But it's clear that government revenue from London road users is much higher than the profit from London Underground.
So it seems strange to assert that roads are paid for mostly by London Underground.