This is a linear route, so you will have to plan your journeys accordingly.
Exeter St David's Railway Station/Wells/Durrington/Stow-on-the-Wold/Buckingham/London Marylebone Station.
Here is the calendar routesheet. Read it even if you intend to use gpx.
Here is the gpx track. Controls are waymarked. Note that rerouting via another app will probably cock-up the route through Wells (it goes over a footpath to pass the Cathedral).