Rob Hamadi wrote:
What about a ticket, eg KXTL-Gatwick-KXTL, getting off at Farringdon
on the return leg.

A ticket from Gatwick (or anywhere in the south) to London Terminals
only gets you as far as City Thameslink. You'd need a slightly more
expensive ticket to get to KXTL (or Farringdon), to cover the cross
London journey.

U