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As harsh as it sounds but altering a destination is amount to a ticket irregularity and you could end in more trouble. You’ve been given a complimentary ticket and the amount of times I’ve seen these defaced with extended dates or change of destination. Best practise is fill in the destination...
However if that person wasn’t challenged at any point to buy a ticket and leaves King Cross station through the open barriers then doesn’t that amount to fare evading
Also begs the question if no member of staff from LNER had come through the train at what point would you have bought a ticket. Therefore the intent, potentially to fare evade you already started the process. I think you got off very lightly. You’ve paid the fine lesson learned, now move on