It depends on what you mean by "ruling." William certainly remained the duke of Normandy until his death, though he spent relatively little time on the continent after the conquest. But if medieval kings and princes had to be physically present for anything to get done, very little would have happened in most places at most times. When William was campaigning in northern England, he certainly wasn't directly administering the city of London. Like almost all leaders throughout history, William had lieutenants who administered in his stead.