On SWR has been going on for years. From memory a lot of the suburban area was 660v until the 4SUBs were withdrawn around 1983. Redevelopment saw the loss of some the original 1915 era huge brick substations (eg Kingston), these were originally manned and had rotary converters from ac to dc.
Battersea-Waterloo was upgraded for Eurostar about 1993. Can see the extra transformers adjacent to the original substations. Since then most of the suburban substations were upgraded, some were done based on condition over the years, but there was a splurge of remaining upgrades ahead of the 10car scheme.
I am not up to speed, but I suspect most of the remaining weaker power areas are probably on less heavily used sections originally electrified in 1930s. Which for normal 701 operating area will be Staines-Reading, Ascot-Guildford-Haslemere, and Farnham & Alton branches. But maybe few other pockets eg Guildford new line or Shepperton branch etc. It could be that power limits on 701s could be set at 10-20% higher power if just few restricted areas were improved, but I don't know if the power limits on 701s are based contractually on substations at the 2017 order date, or what the current network and reduced timetable could handle.