So a solution exists to issue pre-purchased tickets on the train, but the TOCs▸ choose not to use it (perhaps due to the cost of upgrading the machines?)
Edited to add link
Or do what they do in Norway with advance tickets. The Guard prints them all out at the start of his turn and as he passes down the train he hands you your ticket in exchange for a look at your booking print out and your credit card.
If that is too dificult then I would favour the current "grace and favour" system and it
FGW▸ are worried about fraud then why not just have the guard start his day with a printout of all the day's advanced tickets from stations on his line. If your ticket isn't on the list (or he has forgotten the list) he can call control to have your booking print out verified if he is suspecious.