I think this sums up what operators can and cannot do:
The Ticketing Settlement Agreement starts with the concept that there should be at least one inter-available fare for each journey on the national network and that each route has a lead operator that sets the inter-available fare. The lead operator must offer at least one fully inter-available permanent fare on all times on each route. They are not permitted to offer dedicated fares, unless these are first-class, or advanced purchase, or temporary offers. Secondary operators can offer dedicated or joint tickets at any time.
It goes on to say based on when the company was still GNER but the rules are still the same as in The Ticketing Settlement Agreement
GNER can only have dedicated tickets for first class and advance purchase tickets, or temporary promotions not exceeding 84 days in any 52-week period. GNER cannot have a dedicated walk-on ticket, with the exception of first class tickets.
So basically NXEC can only run this if it was a first class fare, advanced purchase or a temporary offer and it is obviously the latter.