Southern seem to have not read these instructions on the ATOC page, as
I have numerous problems with this issue.

I travel from Brockley (South London, near New Cross) to Kings Cross
Thameslink. A return fare for KXT costs £1.60, the same as a return to
London Terminals (that's with railcard discount). Herein lies the
problem. If you go to the ticket machine, and request a KXT ticket,
you'll be confronted with two options - KXT, or KXT (not via
Underground). Both of these cost £1.60. I generally choose the first
one (as it implies you can go via Underground... is that true?).
However, when it prints this ticket, it prints out a London Terminals
ticket. If you do choose the not via Underground ticket, you do get a
KXT ticket.

Sometimes I go from New Cross, or St Johns, both on South Eastern,
rather than Southern. Their price for KXT is different (15p more than
London Terminals), and therefore it doesn't have the problem of
printing the London Terminals ticket.

I assume the problem is that Southern haven't programmed their
ticketing system properly, which is why I get a ticket for London
Terminals. Fortunately, the gates at KXT will more or less let anything
through, shown by my getting through once despite having put my return
ticket in rather than my out part!