Dear Sir/Madam,

In accordance with Directive 2004/38/EC, those family members of EU citizens are not required to go through a visa application process to visit the EU; you will need however to provide with documented evidence of your family relationship with the EU citizen, so the Spanish embassy allows you to have a visa issued. In your case you will need to provide with your birth certificate where it is indicated your father is British, the document must be duly legalized ad translated into Spanish.

Regards,