I know we've done all these arguments before, but..
The timetable is the best place to start with these things. With Greater Anglia's timetables, there is always an indication of whether a train conveys first class accommodation or not. This indication is the familiar '1' symbol. Accompanying text tells us that 'First class accommodation [is] available' on services so marked. GA go to lengths to ensure that only trains where first class accommodation is available are so marked. Trains without first class are not marked by the '1' symbol. Where these services are operated with segregated trains it is often the position of both passengers and staff alike that there is still no first class accommodation available on these trains. First class is not 'declassified'. Announcements are made to the effect of 'Passengers may sit in first class', but the word 'declassified' is not used. This would give authority to the 'Holy Timetable' argument.
..it would, but for the fact that people empowered to check tickets have been known to disagree with this. This includes people employed by Abellio and not employed by Abellio. The Disputes and Prosecutions forum contains a few examples. On the Greenford Branch, operated currently by GWR, such a situation has been avoided by the placing of signs around the first class area noting that passengers may use it. Sadly I cannot remember the particular wording. Whether it was words to the effect of a permanent declassification, or acknowledgement of the lack of first class accommodation, would make all the difference.
As a final TOC example, some trains run by London Overground contain a 'first class' section. However there is no segregation on London Overground. Passengers are thus always entitled to use first class areas. This has never been an issue. To that end, and to my mind, these three examples lend almost nothing but credence to the idea that the timetable is definitive. A few minor misplaced Penalty Fares here and there do not negate this.
The introduction in October this year of condition 15.1 of the NRCoT could well change this. No supplementary 'information boxes' are attached to any part of condition 15. Therefore all of it is intended to be and will be contractual. The particular contention would be in the wording of 15.1: "Where first class accommodation is available, the relevant seats and area(s) of the train will be clearly marked." Does the marking of an area as first class only take effect when first class accommodation is available? Or is first class accommodation only available where marked? Do markings negate our otherwise 'authoritative' timetables? To me, first class is only in existence on a train where 'available' as in the timetable, and then 'marked' as in the NRCoT. The NRCoT seems to draw its wording from that contained within timetables. It would be most helpful for the NRCoT to clarify this for us.
It is very easy to argue that where first class accommodation is 'marked', it then becomes 'available'. What about in the situations where a first class carrying train rocks up to form a standard-only service? Does a portion of the train immediately become unavailable? Should train crew, station staff et al. have to make explicit at all hours that there is no first class, or should they be able to rest knowing that this particular train never carries first class anyway? What happens in situations where a TOC 'borrows' a train from another, permanently or otherwise, where there are first class areas marked, but they do not wish to carry first class passengers? Need they also go to the huge effort of letting passengers know that there is no first class, or should they stick with the sensible approach adopted to by LO and realise their customers will figure it out, by such methods as 'looking at the timetable'? I do not disagree that TOCs should get off their arses and remove any such demarcation, but in the interim a solution must be available.
Using the NR website to ascertain the availability is a different kettle of fish altogether. Again, the new NRCoT, in condition 20, gives hope to those that abide by the all knowing wisdom of the internet, even if it makes reference to an entirely different situation and is not really explicit enough.
The best way to resolve all of this, of course, would be the words of a statement issued by ATOC, the DfT etc. (barring anything sensible like clarifying in their contracts). Does any such statement exist? Comboios de Portugal are able to be very clear on this. Only certain trains have first class. All trains of these types have first class. If you hold a standard class ticket you will damn well not use first class until you pay for it. There is no such rubbish involving the timetable for determining which trains have first class. Great Britain should be able to be just as clear.