As early as January 1962, the Foreign Office had notified Madrid of Guerrero's assignment to Spain, which was accepted late that month. He was in Madrid in May the previous year when he delivered a lecture on the occasion of Rizal's birth centenary. Delivered in the midst of writing Rizal's biography, his lecture dealt with Rizal's Hispanism as reflected in his novels. Those novels, he said, were a dialogue between two ambivalent Rizals. Rizal had a love-hate relationship with Spain, but this ambivalence gave birth to his love for his beloved nation; he created a nation because the Spanish nation repudiated him.

A week or two before his departure from London, Guerrero and Annie packed clothes, books and research materials to be brought to Madrid. Annie donated five parcels to the Save the Children Fund. Then, she fell ill, exhausted from all the packing, and was brought to the Priory Clinic. Hearing about Annie's illness and their departure, their close friends and acquaintances wished her a speedy recovery and thanked Guerrero for the outstanding services he rendered to the Philippines and Britain. After Annie's recovery, the couple attended an official farewell lunch given by the Marquess of Landsdowne, the foreign undersecretary of state, and his wife. Instead of a formal lunch, usually given for arriving and departing ambassadors, the marquess agreed to make it informal among close associates of the couple.

While preparing for the next post, Guerrero thought it was opportune to be awarded the Knight Grand Cross of Rizal before the presentation of his credentials but in an unprecedented way. He had the Grand Cross of the Lion and Grand Cross, Order of Dannebrog from the Finnish and Danish governments respectively for meritorious services as ambassador to these countries. He was a Knight of Rizal. Now that he had his prize-winning biography and translations of Rizal's novels to his credit, he thought he was qualified for the award. Upon learning later that the unorthodox way of bestowing it would create a controversy with the oldest members, he gave it up as mere “vanitas vanitatum”. But it was not a lost cause for in another assembly of the Order in the same year, his nomination was approved, with the awarding ceremonies to be made in February the following year.