A couple of months ago I turned up at Kings Cross with First Class tickets for the GC service to York. It was cancelled. (I suspect an overrunning Engineering possession had delayed the incoming stock, although no explanation was given)
However, in good time before the scheduled departure, two GC staff arrived at the allotted queuing point, and issued replacement tickets for NXEC, and we were able to join the service that left slightly earlier than the scheduled GC departure time.
I noted that although it was a Sunday, we were given full open first singles rather than a standard ticket with weekend first upgrade, which I presume would have been valid, and much cheaper for GC.