I have what I imgine to be a similar commute on the Devon Somerset border, 25k , a few metres climbing.
At the moment I do it on a single speed. Only trouble with this is that the quieter route home would be a bit painful so I use the main road. A single speed is a great commuting bike choice for all the reasons I'm sure you've seen before of no expensive bits to wear out and simple maintenance.
I use a 66" gear most of the year, but a 70" was fun in the summer.
Apart from the no gears, it also has marathon+ tyres, 36h chrina ridga, shimano hub generator and b&m IQ Fly light.
I am currently building up a replacement bike which will have gears.