You should have hoped for a friendly passenger. The King's Cross - Cambridge train is often 'standing room only' . Once, someone came into the first class section I was in, the inspector demanded a penalty and extra fare from them, and I told him that it was obvious that there were no seats and the train was overcrowded, I didn't mind, and he could, perhaps, see his way to forgetting about it. He abandoned the idea. It is within the power of 'the train manager' to let people into first class anyway.
Just imagine what the litigation could be if people in an overcrowded train were injured because of it, say in an emergency stop, or simply because of the overcrowding itself. No judge would be listening to an argument beginning "we only sell a journey, not a seat". Even Ryanair couldn't get away with that !