In my experience there has been an iron reluctance to organise ticket acceptance, time and again, when I have been using a ticket valid only on Northern services. Time and again my Northern train has been cancelled, and staff have been asked if acceptance with TransPennine Express, East Midlands Trains or Transport for Wales can be arranged to lessen the delay. Time and again I have been refused categorically, resulting in 2 hour or longer delays to my journey in many cases. Journeys include Manchester to Liverpool, Sheffield, Leeds, Crewe etc etc. This all changed last May, when the eyes of the industry turned to Northern, and automatic permanent ticket acceptance was put in place, ostensibly for any delay, so including even one of 10 minutes. This has now expired of course, and I don't know if attitudes have changed much.
On one memorable occasion I had a London to Stoke-on-Trent West Midlands Trains only ticket, and was told at London Euston that London Underground to London Marylebone, Chiltern Railways to Birmingham and then CrossCountry to Stoke-on-Trent would be permitted as there had been a fire and no trains could run from London Euston station. There was no ticket inspection on the way to Birmingham, but the train manager on CrossCountry's train said that the ticket was not valid because it was routed West Midlands Trains only. I explained that ticket acceptance was in place, but he said it had not been organised. I refused to pay for a new ticket from Birmingham to Stoke-on-Trent when this was demanded, so I was asked to leave the train at Wolverhampton. I refused to move, was therefore taken to Stoke-on-Trent, and nothing else ever came of it. I complained to CrossCountry, who replied that the Train Manager had been correct, and that it was my fault for buying the wrong ticket, as no ticket acceptance had been arranged. I also complained to West Midlands Trains, who did not comment on any of what I had told them, but did pay me compensation for the full value of the ticket.