It still isn't that likely. What option does the guard have, in reality, outside of the major cities? Particularly if it's a local journey? Most TOC's would not want to be hit by fines for late running trains in order to collect a £5 fare. TOC's also tell their front line staff to avoid confrontation, and not to risk their own safety.
The reality is, that faced with an uncooperative but polite passenger, who insists they have bought a ticket but can't find it so they will not buy another, the options available to the guard are quite limited. They will normally use their training and communication skills to persuade the person that they need another ticket, should they continue to refuse, there is often very little that can be done.
Naturally, there will be exceptions when BTP will be called to deal with ticketing irregularities, but they will be very rare.