I don't understand why they don't introduce phone voting as an option. Just a reworked a version of any telephone banking that's been around for decades would do it. You get sent a pin in the post or email and when you call in you can change it. After you've cast your vote you can call to verify it is as you chose and not been fiddled with, or even change it later up to a certain cut off time. Not 100% bullet proof obviously, still some could be coerced to vote a certain way in some households, and the deaf wouldn't be served by it. But most people including the elderly can always access a phone from anywhere.