I can't be bothered to trawl but look at the SWT site and it will state very clearly the Ts & Cs - so Southerns are different, believe or not a different company so each one has a different set of Ts & Cs.
I expect each airline has a different set of Ts & Cs as well but let's just ignore that shall we ?
OK - I've done a search of SWT web site as well. Nothing about terminating your journey early.
So, two train operator sites down and nobody has managed to show that there really is such a T&C. I'm all for sticking to T&Cs but they do really have to exist to be valid !!!!!!!