You are correct that the surname Rey is very common in northern Spain and France,but it is also found thruout the old cristian world. During the pilgrinages to Santiago the Compostela, which was a once in a life time requirement of all able cristian males in the middle ages it was the custom to award the title of Rey (King in english)to the first of the pilgrim group to see the towers of the Cathedral of Santiago, as the aproached the city on the road of Saint Jaques (Camino de Santiago). The hardships found in travel in those days were enormous and Pilgrims traveled in good size groups for support and protection. The sucessful completion (and return from such a journey) was considered a great honor. Many apparently kept their title as part of their names.