The French article says that the Prince and Princess of Wales were guests of honour on a reception in London given by (the Spanish) Ambassador and Mrs Puig de la Bellacasa.
By the way, Mr José Joaquín Puig de la Bellacasa y Urdampilleta (the Ambassador) comes from an ancient family with strong monarchist roots. Mr Puig became very close to Don Juan de Borbón and even more closer to Don Juan Carlos. On the picture below you can see Mr Puig in jacquet behind Doña Sofía and Don Felipe.