Aside from the ~£30b raised in fuel taxes, there's also the ~£6b raised in vehicle excise taxes.

Both of those revenue streams go into a government general fund and are not allocated specifically for road maintenance, because they collect about 3x what is actually spent on highway & road infrastructure each year. [1]

In short, the roads & highways are massively profitable.

London's buses however, lose nearly ~£1b a year, which tanks the entire public transportation budget, and more than offsets the profit generated by the Underground.

[1] - https://www.racfoundation.org/data/road-user-taxation-highwa...