Do you mean administratively or culturally? I’d say that all of them are edge cases, but Sanderstead is the “most” Londonish of any of the places you’ve listed.
If you’re looking for whether the services “should” be provided by TfL or “the railway”, Sanderstead is then the least Londonish because it’s on the Oxted line and served as part of that route, not as an inner suburban branch like the others.
To illustrate the point about fares, administration and culture, I used to live in Orpington. “For” it being in London, we were in a London borough (Bromley), zone 5, and London bus routes. But we also had a “BR” postcode and 01689 telephone number, and our address was “…Orpington, Kent”.
If we went out a couple of miles to Pratts Bottom, Knockholt, Cudham, or Biggin Hill, we’d definitely left the built up area and were in Kent - but still in the London borough of Bromley. The boundary was and will remain blurred.