The consultation area is 120m wide, not necessarily the trenching. The working width is often far less than that: https://www.nationalgrid.com/document/340431/download

In this drawing, you can see the area in the map and it is not 120m wide along the trench: https://www.nationalgrid.com/document/357086/download. For scale, the grid squares are 1000m.

A 400kV trench construction swathe also includes the soil storage areas - subsoil and topsoil are separated for return afterwards, as well as clearance to the fencing (https://www.nationalgrid.com/document/357086/download).

Why do trenches need to be dug across the countryside? Put them alongside existing roads and rail lines. Same with above-ground power lines. It might make them a bit longer, but the ‘eyesore’ is already there, and we can avoid making new ones.

(Re rail lines — if you build power lines over existing rail lines you could also electrify the rail route at the same time, and get rid of the diesel locomotives).

To be fair to the National Grid there - a 400kV power line is substantial: it has to have phase separations and be buried deep enough, plus space for reactive compensation from being buried.

Roads also go to places with buildings and have junctions, plumbing, foundations and are generally hard to dig past. But there are places where they do follow motorways: https://commons.wikimedia.org/wiki/File:M6_motorway,_northbo... (also canals).

Rail lines go though towns by design, and as you see from comments even here, the one thing people really hate the thought of is power lines near houses.

120m would be an absolutely insane width for a trench. It seems more likely that you’ve misinterpreted that.