The UK's National Grid has a useful document on underground cables.[1] There's a newer insulation technology that's replacing oil-cooled cables. Cross-linked extruded polyethylene ("XLPE") is used as the insulator. NKT (Germany) and Sumitomo (Japan) sell underground cables good for up to 500KV. The technology is about 25 years old now.
So the Scattergood-Olympic line mentioned in the article has been replaced with a more modern cable. The project was completed in 2018.[2] The old oil-filled line is being kept as a backup.
[1] https://www.nationalgrid.com/electricity-transmission//sites...
[2] https://www.ladwpnews.com/ladwp-completes-construction-of-th...
