I say again, this is a Stevenage to Kings Cross commuter, not someone who mostly goes down the Wimbledon branch. From the Great Northern (as that is actually the operator of the service) website
finding it hilarious how confusing and passenger-hostile the railways are that on a forum of train obsessives it took till post #50 to point out that the entire discussion is wrongly titled, and much of the accompanying discussion is also wrong, and GTR themselves didn't even point out that this wasn't a Govia Thameslink Railway Thameslink train, but a Govia Thameslink Railways Great Northern train.
Train passenger fined £80 for sitting in wrong first-class carriage
Thameslink policy is confusing and should be scrapped, says angry teenager
A Thameslink spokesman said: “Rear first-class compartments on Thameslink trains have been declassified since they entered service in 2016, to create more space for customers with regular tickets. We advertise this on the digital displays on board the train, as well as on platform screens.
If GTR don't even know which of their trains are which how on earth are passengers supposed to?
Absolutely indefensible.
The even funner part is there ARE Thameslink trains, at 0417, 2317, 2331, and 2351 only, on this route, which they don't bother listing on their 'first class' page but it should be obvious to GTR at least, that the passenger was not on a Thameslink train but a Great Northern train so the answer they gave to the media was irrelevant nonsense.
(Not really surprising the passenger tweeted @TLRailUK rather than the correct @GNRailUk, given that the penalty fare is labelled 'Govia Thameslink Railway'.)