electrification of chiltern mainline would be a challenge especially around London area. It looks like a lot of changes to bridges and tunnels would have to be done at a high cost doubt anyone wants to pay that cost anytime. Maybe hydrogen trains could be used as kind if trial to replace mk3 then if successful then all over chiltern mainline as replacement for class 165 and 168.
Hydrogen isn't a sustainable large scale option. It is probably the best option for "long, thin" branch lines like the Far North but not for a major London commuter line. Producing it requires far more electricity than the energy it provides.
If there's a bridge/tunnel issue, discontinuous electrification using batteries is a reasonable fix.
Aylesbury via Amersham is a more interesting one. As the Met isn't going to go OHLE any time soon, serious consideration should be given to a specific solution that can run on both fourth rail and battery, potentially with some sort of charging mechanism (e.g. a short length of switched fourth rail, shielded and only active when a train is over it) at Marylebone and Aylesbury/Vale Parkway. That, to be fair, would free up enough 16x to replace the Mk3s. If there was a failure they could always chuck a DMU out instead, so there's backup for what would be a microfleet.
But that aside, it is ridiculous to have a non-electrified mainline into one of Europe's largest and most important cities, connecting it to England's second city. It
must be electrified.