Maybe all postal deliveries should be done after dark - so the EVs have all day in the sun to charge?
All day? In winter? In the UK? I take it you've never lived here.
In a UK winter a PV panel has an average capacity factor of around 5%, and may dip below 1% for days on end if there's no direct sunlight (and zero if there's snow on the panels). Even for one day's power for a delivery van, the required area of panels would be immense if it was to be reliable.