Well, they're stretching a point, because the reality is the full, undiscounted fare in Scotland is now the off-peak.
If they create a new fare solely for the purpose of penalising people for not paying before boarding, whatever they call it, that's not the "full, undiscounted" fare, that's a penalty fare. For it to be the "full, undiscounted single fare" as per 9.2.1 NRCoT, there would need to be a circumstance in which it would be charged other than to penalise somebody.
All the above is in my humble opinion, of course. Not legal advice.