My understanding is with an outboundry travelcard you can take an outward journey by any permitted route (subject to any route restrictions on the ticket) that does not pass through the travelcard area to any station on the boundry of the travelcard area. Then you can perform unlimited travel within the travelcard area, and finally you can make a return journey from any station on the boundary of the travelcard area (not nessacerally the one you entered at) back to your starting station via any permitted route that does not pass through the travelcard area.
If you were travelling via Leatherhead you would enter the travelcard zones at either ewell west or ewell east. The corresponding routing points are Epsom (for both), Motspur park (for west) and Sutton (for east). Epsom seems the most appropriate routing point here.
The list of maps for Reading to Epsom are LONDON AR WX+DK WX+HR XR+DK,
"LONDON" is obviously of no use to us.
"AR" allows travel via Workingham, Guilford, Dorking and Leatherhead.
"WX+DK" does not appear useful to us as all the routes it permits seem to go through the travelcard zones.
"WX+HR" does not appear useful to us as all the routes it permits seem to go through the travelcard zones.
"XR+DK" allows travel via Workingham, Guilford, Effingham Junction and Leatherhead.
Looks good to me. I was also able to get the journey planner to offer me the outboundary travelcard on journeys from Reading to Ewell East via Leatherhead.
As the North Downs Line exists an out-boundary ANY PERMITTED Travelcard can be pretty flexible. But it's a quirk, as the ticket offer is clearly designed to use radial routes into London.
Entry points into the Travelcard 1-6 zones I can think of from my local North Downs Line station via permitted routes (shortest distance or mapped):-
West Drayton - via Reading
Feltham - via Wokingham and various other combinations
Surbiton - via Farnborough North/Main, North Camp/Ash Vale and Guildford
Ewell West - via Guildford
Ewell East - via Guildford
Coulsdon South - via Redhill
Knockholt - via Tonbridge
Also, travel to some SW London stations is valid via Dorking Deepdene/Dorking Main.
So, routes via Berkshire, Surrey, Sussex and Kent. Not bad!
A question. For the more "interesting" routes that wouldn't be permitted on a ticket from my local station to London Terminals, do I have to take services that stop at the Travelcard boundary stations?