This zone would extend as far as Kingston, and some people don’t live near their school or a reliably serviced bus stop
In Bromley (which is also getting the ULEZ extension) the buses go pretty far into the sticks.
Places like Keston, Downe and Biggin Hill have good TFL bus access while being basically rural villages. Seriously, look up Downe on a map. It's in the middle of nowhere.