Yorkie - You say GC and NXEC should obey the laws they are given, yet you say to passengers that they should break the railway byelaws and that you would too, this is, frankly, hypocritical. Not meant as an insult, just a fact.
The byelaws are quite specific, you cannot board a train with out a ticket unless there was no facility to get one at the start of your journey (unlikely at York), there was a notice on the station to that effect, or that an authorised person gave you permission.
It is that simple. It can't be argued with.
You could, in theory, go into the ticket office of GC (Max says there is one at York) or infact NXEC, and get them to tell you that you can buy onboard, and get their details as perscribed in the byelaws, but that would really be pointless as you might aswell buy the ticket there.
Max - A sign on the departure board would give you permission to buy on the train so it does seem strange that the notice would be on the boards at Kings Cross and not York.
However, the Byelaws do say "A person acting in the course of his duties, who is an employee or agent of a train operating company". A website, no matter how good, could never be a person, even if it was an agent.