We (the forum) have been down that road before.
I must have missed it!
I'd be interested to see any evidence you have to support the claim that this is "quite likely".
How many of them do you think there are! I can only report from my own experiences, which tend towards tolerance in the case of asking before boarding. The one exception is Northern, where I now don't bother because it's either a "No of course that's not valid, why are you even asking me." or just profound confusion about the question, or maybe "Yes this train goes to X.".
Anyway, I'm not quite sure how your approach tallies with what most people call 'common sense', but if someone has a ticket that is from Manchester Stns and to Sheffield, and is routed Any Permitted... why wouldn't that be acceptable to most guards?
Maybe, I haven't looked at the figures, it was just an example.
Of course, fine in principle for a longer journey.
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I had an excess from a TOC-only ticket to an unrestricted one when ticket office clerk issued the wrong type and I said that I'd been given the wrong ticket. Should I have asked any guards before "trying" to use a ticket that's only invalid by rules internal to the railway industry, nowhere published; and that's not the same as the one I asked for?
It's very clear that should such an excess be issued, it may be used.
So in one case I'm asked for evidence that the ticket would be accepted if a consultation before boarding took place, in another I'm asked why there should be a question mark over it's validity in the first place.
You can hammer out the details if you like - the point I'm making is that the passenger has paid the same amount as someone who bought the Any Permitted Anytime Day Single in the first place (plus possibly an admin fee). There's very little justification for the rejection of such a ticket therefore.