I can't see Greenford being wired until the Chilterns are, and even then it's a really marginal case.
The 4 GW Branches (Greenford, Marlow, Henley, Windsor) could all be operated by a fleet of 40m units with Battery/OLE capability. Stadler or CAF would probably be quite happy to adapt a design for that length. That would give you a reasonable critical mass as you could have something like 8-10 units (1 Greenford, 1 Marlow, 2/3 Henley, 2/3 Windsor, 1 Exam, 1-3 spare) to respond to demand. Electrification would then be basically as far as was easy from the main line until either locals got too noisy or you reached a problem bridge.