@slowe it's free to download here, without registration:
https://data.ssen.co.uk/@ssen-distribution/distribution_line_substation
A number of them now require registration but it's all still CC-licensed once you're registered...
Shape file data containing our geographical position and attribution of the electricity network covering the SEPD and SHEPD DNO areas. A link is also provided for the Electric Office Web Portal for Independent Connections Providers (ICPs) to to view, query and print map based GIS Data, showing the location and associated data for circuits, cables and overhead lines across the Distribution Network at all voltage levels from EHV -> LV to assist in the planning of connections. Object information is drawn from EO to show selected asset details such as operating voltages. _These maps cannot be relied upon as they are indicative only and while we make every effort as to their accuracy, the position of any cables may have changed. In addition there may be private cables which are not shown on the plan. Before carrying out work of any kind within vicinity of any of the cables, you must carry out a search on Linesearch https://lsbud.co.uk/_