'At York' as you have rightly remarked is far away from the scene of action. But York is figuratively used to mean a distant place; It metonymically describes any place far off, particular for the general.
Lizzy doesn't care whosoever is around, she's oblivious of the surroundings and given to follow her own sweet will, all are removed from her reckoning to places as distant as York. Jane Austen chose York, she might as well choose any other place to suggest the character's detachment from the persons around her.