Cables on overhead high voltage lines are mounted using stacks of ceramic insulators, but here they seemingly just sleeved in some protection and hang on a tunnel wall. Why is that?
The cost of oil insulated cables that can do 132kv is about £900 a meter. Whilst there are HV cables that exist on the outskirts of london, they are much rarer in zones 1-3.
I assume that the cost of pylons with raw cables is much much cheaper. The problem is planning permissions, physical clearance. planning permission and now one wants to live near HV cables (that they know of. There are a bunch of 33kv cables buried outside posh people's houses in zone 5, and a bunch in canals.)
That means the path through the air to some conducting materials needs a certain distance, and that even when wet or iced over or whatever can happen up there.
Here’s some high-voltage cable spec sheets that show a cross-section of the assembly for voltages above 69kV: https://www.southwire.com/wire-cable/high-voltage-undergroun...