Is there any rover ticket you can get to travel on the sussex and kent routes, a bit similar to the North East All Line Rover?
I looked on GTR's website, but can't seem to find anything.
There's the
Southern DaySave (£21), which is valid across all of Southern's network with certain time restrictions. Note that this must be bought online at least 3 days in advance.
There also used to be a
Thameslink DaySave (£23), however it's not currently on sale anywhere (you used to have to go to a tourist information centre in Brighton or Luton to buy it IIRC!).
You can also get the
Kent Rover (£45 for 3 days, also available for 7 or 14 days). However this only valid in Kent (as the name suggests), with Swanley being the only station where the area of validity adjoins the Oyster/contactless PAYG zones. It's also not valid on HS1 or Thameslink services.
The SE is a bit of a desert as far as rangers and rovers go, unfortunately!