As I say regularly, but will say again, the VAST majority of people I sell tickets to would be described as "Pay when challenged". They will have come from a manned station, with a booking office as well as a TVM, but an unbarriered station or platform. They will deny knowing they had to buy a ticket, despite walking past posters in most stations saying "Buy before you board". They will not be paying with RTV's, postal orders, or those plastic things from the local council nobody ever knows how to process. They will frequently have a card that comes up "Declined" and will then say "It never works on-train". When asked why they didn't buy before boarding nine times out of ten they will say "I was running late", regardless of where or how frequent the train is, or how long before departure they boarded.
I sell a few hundred tickets to these people every month. Pretty much every one of them is in breach of Railway Bylaws, but manages to suffer no consequences. They still get their Super-Off-Peak with railcard discount because the "money-grabbing TOC who threatens prosecution as a mechanism to extract extortionate out-of-court settlements" who employes me will have my ass if I try to sell them a full open and the passenger complains or kicks off.
I'd love to know which TOC it is that is trying to catch passengers out with unfair underhand ticketing. I'd consider working for them. In my experience, there are many many more passengers knowingly playing the system, and TOC's tolerate them as the whole system is geared up to these peoples "Rights" under the guise of "Customer Service" than in making them pay for the service they are receiving.