Yes, the Fleet River Sewer still flows under London, the residue of a time long past when it was a river, and later a destination not just for detritus and waste, but also for the people who earned a living in its dank, smelly waters. So-called mudlarks and toshers scavenged items of value there, including bits of coal that had dropped off barges. Often these people were small children trying to eke out a living underground.
An interesting complex of tunnels and vaults are to be found close to the Roundhouse at Camden. The Camden Catacombs, as they have become known are were once owned by British Railways but have now passed into multiple ownership. Some sections were demolished during the redevelopment of the area while other sections belong to Camden Market who discourage access