Heard battery fitted class's 350/2's were being looked at with them fitted with shoes to use 3rd rail.
The issue is the long gap from Bassingstoke to Exeter. A push to extend 3rd rail at least to Salisbury was bring considered I've seen mentioned elsewhere
Significant expansion of conductor rail schemes whilst in theory just about permitted is de facto prohibited by the elfansafety industry.
Perhaps a case for 750 volt
DC▸ overhead electrification ? To Salisbury ? The reduced power available at this lower voltage could be supplemented by power from the battery when needed, with he battery being charged whenever the train is stopped, coasting, or otherwise not needing full power.