IMHO the status of Farringdon as a non-London Terminal is a ridiculous
anomaly that catches out large numbers of people who believe themselves
to be holding a valid ticket. It's ridiculous that with my particular
season ticket I can get off the train at City Thameslink or KXT (a good
deal further on than Farringdon and no more of a "terminal" under
non-blockade circumstances), but if I dare to get off at the station
between them I risk a £10 penalty fare. Grr.

Kings Cross Thameslink is not a valid "London Terminal" for journeys
from the south.

From the Thameslink website:

http://www.travelbuddy.info/main.php...id=6&page_id=9

"Customers travelling to London from the south, including the Brighton
line and the Wimbledon – Sutton – Carshalton line, will continue to be
issued with tickets with the destination “London Terminals” if
travelling to London Bridge, Blackfriars or City Thameslink. Customers
travelling to Farringdon or King's Cross Thameslink from the south
will continue to receive tickets naming these specific stations."