It will likely need to be tri-mode overhead/3rd rail/battery, which does not currently exist in the UK, so new trains would be needed.
Overhead may not be needed given than it would be only used for a short secton north of Acton Central, and possible for recharging at the northern terminus. However 3rd rail has power limitations so may not sufficiently charge the units in the time spend there.
This assumes that electrification does not take place, which I feel is likely given that the current 3rd rail policy would mean a 25kv island from South Acton to ~Kew Bridge, as well as other necessary electrification work. Batteries are probably much cheaper.