There are plenty of loopholes within the Southeastern network which I wouldn't hesitate to use if I had any reason to travel on their trains.
I would excise caution however. A little knowledge is a dangerous thing - few Southeastern staff seem to understand how to find out what a ticket entitles the holder to do. Southeastern have plenty of time served staff who can offer an educated guess based on what sounds right in their own mind. Those who can read and understand the Conditions of Carriage and Routeing Guide, then apply the information within to establish the validity of a ticket, seem to be very thin on the ground.
This lack of knowledge is combined with what seems to be an unwritten zero tolerance policy against people finding legitimate ways of saving on through fares. Southeastern consider this to be fare evasion - they'd sooner report it to the police with a view to securing a conviction, than accept that permissible saving opportunities exist and deal with them as appropriate. There are anecdotes on how Southeastern's front line staff and management have dealt with ticketing issues elsewhere on the forum.
You've been warned