Overhead conductors use air as the insulator. Underground cables use an insulating jacket. In the past it was really difficult to build cables with voltage ranges in the 10s of thousands of volts without additional complexity like a dielectric oil being pumped through the cable. I think modern dielectrics are significantly better though.
Yeah, the wires in the new London tunnels are XLPE. Despite being first used in the late 60s it took a long time to be commonly used. Though much of the infrastructure around is still very old.
