Yes, but people who call them tube tickets aren't normally using them from stations with no tubes for the past 10 years If someone knows that they need that ticket for use on their train, it does seem a bit odd to not attempt to buy that ticket until you have already done part of the journey on that train, on a route that makes it very clear you must have a valid ticket before boarding. I reckon the RPI thought this story simply does not make sense.
I agree it's odd. But there's nowt as odd as folk. Or something like that!
I
ndeed, PFs are intended for people who make mistakes and should not be issued to fare evaders, which is a "completely different matter" as documented here.
Well, it appears that Southern think this is a case of fare evasion, which is why they have sent a letter about prosectuion. Which brings me back to the OP having spent more on tickets than they needed to.
The OP is definitely guilty of travelling without a valid ticket though.