Hi Luke, Welcome aboard!
Rural distribution goes back to before the 1940s, although I have to confess that I couldn't tell you the first HV line installed.
In the earliest days of mains electricity DC generators were used, which of necessity had to be located very close to the areas they served due to power being generated at the same voltage at which it was utilized. That's why the old districts of some cities still had DC power as late as the 1950s, as they were the earliest areas to be wired for mains.
HV transmission lines only became practical with AC generation. I would think that the earliest HV lines were what would today be regarded as short haul -- Just distributing AC at "low" HV voltages (3.3kV etc.) around a city. The more extensive network which eventually became the National Grid grew up gradually from that.
I don't doubt that the very early poles and xfmrs looked somewhat different, although the U.K. seems to have settled on common designs from a relatively early time: Horizontally aligned cables on cross arms for 11kV etc.
Some of the early equipment would have been different by necessity though. Back in the 1920s and before, some areas were running on 25Hz AC instead of 50, which would mean much larger transformers for a given power.