The dispatch process is different for 80xs. On an 80x the doors are opened by the driver, closed by the TM, and then whilst the TM is keyed in to the door panel the orange lights on the side of that carriage will flash. The other stock they primarily dispatch are voyagers for XC which use 10 bell dispatch (driver controlled, but guard instructs driver to close the doors when clear).
There is however still one small fleet serving all of those stations North of York that does have the same dispatch procedure as the Mark 4 sets, and that is XC’s sliding door HSTs. The guard is in control of everything on those, and as with the Mark 4 sets is capable of observing the dispatch through a droplight window (TGS on a HST, DVT on a Mark 4 set). I am amazed that familiarity with dispatching those isn’t considered good enough to dispatch Mark 4s.
Train crew knowledge would only prevent running North of Newcastle, as opposed to North of York. Although Newcastle based crews no longer sign Mark 4s, there are both drivers and train managers that sign both Mark 4s and the route to Newcastle, as Leeds, Doncaster and Kings Cross crews sign up there. It would be no more difficult to find crew that sign both Mark 4s and Newcastle as it was to find crews to take 90s there a few years ago, if anything easier in fact as only Kings Cross signed the 90s.
The final issue regularly given is the lack of a thunderbird. Although it would make things a lot easier if a 91 were to sit down North of York, it’s not essential to the operation of the sets and the sets can run without a Thunderbird being located anywhere nearby.
Ultimately its self imposed inflexibility on LNER’s part that prevents the Mark 4s from running North of York - none of the three issues are unresolvable, they just cost money and time that LNER would rather spend on talking info screens on concourses that hardly anyone uses.