A court may have to answer it but I am afraid the answer is inevitable which is why three levels of appeals bodies haven't really understood the OP's point. Penalty fares go back to the British Rail Penalty Fares Act 1989. Here is the House of Lords second reading on the Bill and their lordships are unanimous in their interpretation of the expression.
https://hansard.parliament.uk/Lords...e76718a53/BritishRailways(PenaltyFares)BillHl
I think your mistake is conflating "full" with "normal". Of course, the Super Off Peak is discounted. It is cheaper than the Anytime Return.
The super off peak fare is not discounted, it is the fare applicable on the date an time of travel. The discounts that must not be applied are those in respect of railcards.
At risk of going around in circles the regulations set out in Statutory Instrument 2018 / 366 - The Railways (Penalty Fares) Regulations 2018, in force from 6th April 2018 are quite specific and in my mind are not even arguable. They state;
(6) In this regulation “the full single fare applicable” is to be determined by reference to—
(a)the age of the person in question;
(i.e is the fare due appropriate to an adult or a 'child'?)
(b)the day and time of the journey that person is making, has made or intends to make, as the case may be;
(what is the fare payable for the particular service at the particular time on which the passenger is travelling? Peak (anytime) off peak, super off peak etc. If the travel is at a weekend the appropriate walk up fare that a passenger would pay at a booking office window for that journey is the fare that is the reference point for the PF charge)
and
(c)the train and route that person is using, has used or intends to use.
(i.e on a journey from, say, Lichfield TV to London, are they travelling direct on VTWC? are they travelling direct on a LNW service?, are they traveling (more expensively) via Birmingham? Each route and service has a different level of fare due for the train used. This regulation requires that that should be applied - not the blanket anytime / all available routes fare)
The appeals panels, it appears, have misinformed themselves.