Funnily, the primary culprit for the hopeless delay and budget overruns on the Great Western electrification in the UK is supposedly because of their inability to dig holes in the ground and install posts as fast as they thought they could. It finally led to scaling back of the project in CP-5 excluding electrification for the time being of the segments Chipenham - Bath Spa - Bristol Temple Meads, Bristol Temple Meads - Bristol Parkway and Cardiff- Swansea.
They have been having ongoing issues with Class 80x's interfering with signal circuits not only in the newly electrified areas but also on the previously electrified areas, like on ECML, and Hitachi is working overtime to reduce the dirty electromagnetic footprint of their trains, while NR is working overtime for replacing parts of the signaling systems that are inadequately shielded. So electromagnetic issues can affect long existing electrified areas too, as new technology (specially trains with a whole bunch of inverters) are introduced, apparently.
BTW, three phase is seldom converted to single phase when the electrification use standard frequency. They just alternate phases for feeds to different segments of the catenary. But they do supply the power in center tapped 50kV feeders in typical modern 2x25 configuration of power supply to increase the distance between feeder posts. But then there are booster auto transformers installed in between as needed to maintain line voltage.