France was trying to impose Napoleon's brother as king of Spain, whom the Spanish did not want, and they used harsh tactics to put down the Spanish uprising (see Goya's painting "The Third of May"). So it probably wasn't hard for the British to find willing collaborators among the Spanish people.
Moreover, when Wellington carried the war into southern France in 1813-14, he continued the same policy, with the result that the local French people would supply the British, but not the French army!