wrote

Then there's Dartford. It's hard for customers on the 3 north Kent
parallels to get from one line to the other without going back to
Lewisham which might not be in their zone. Is it permitted to change
trains at either Dartford or Lewisham on a travelcard if you promise
not to leave the station?

Not permitted. There are off-peak trains which loop between Crayford and
Slade Green (and these stay within Zone 6, although they cross the Greater
London boundary), and a few peak trains which loop between Barnehurst and
Slade Green. You can of course use a Travelcard to get to Dartford, as long
as you travel on a London bus, and not a train.

Peter