Sorry if I'm stating the obvious here as you do seem to know the area, but I can see the several references to "cross country trains" being quite confusing to those who are unfamiliar with British trains! The train operator "Cross Country" operates no services from Kent, and there aren't any routes from Kent that go across any significant portion of the country - only into the neighbouring counties. You'll need Southeastern until you get to Tonbridge then Southern after that.

I guess you might just mean it goes through the countryside, but then pretty much every train route does. Sorry again if you knew all this - I guess it may look pretty pedantic!