DT17552 - Double Taxation Relief Manual: Guidance by country: Spain: Source of income
For the purposes of the Elimination of double taxation Article, profits, income and capital gains owned by a United Kingdom resident which may be taxed in Spain under the provisions of the agreement are deemed to be income or gains from sources in Spain (Article 24(4)).
Interest and royalties are deemed to arise in the country of which the payer is a resident.

