Issues relating to validity at/to/from/via London Terminals is one of the reasons why DfT/TOCs are keen to push people onto itinerary based booking sites/machines, rather than TVMs or other systems (such as BRFares) where people can simply select the product they want.
If anyone wants to be certain that a ticket covers the full journey in the London area, where tickets may be issued to London Terminals, then my advice would be to use an itinerary based journey planner, unless you are certain you know exactly what you are doing.
If anyone is ever in any doubt about the validity of any ticket that isn't purchased in conjunction with an itinerary, I suggest asking on the forum before making a purchase (the only caveat to that is if it's an anomaly/loophole, then posting it may accelerate its demise!)
A bit off-topic but I've been mis-sold this ticket before from other retailers ...
If anyone wants to be sure the ticket covers the full journey, then I would advise using an itinerary-based journey planner that follows the rules, such as the forum's ticketing site.
....I wanted to travel up to STP so I could get a seat back to Brighton in the evening peak. I'd already set off before I noticed it was issued to/from London Terminals ...
If the forum's ticketing site was incorrectly issuing London Terminals tickets without validity via Farringdon, with itineraries via Farringdon, this would be flagged up as an issue and fixed very promptly! However with some journey planners/retailers, such issues can go a long time without being fixed, if ever at all...