The S1 form is a document that will allow you as a pensioner to request access to the Spanish healthcare system. The application form (S1 form) is provided by the Overseas Healthcare Services from the NHS.
Once you have obtained the S1 form, you need to present it at the local INSS office (Spanish Social Security Office).
If you cannot visit an INSS office in person, you can register your S1 form online instead. Please see details on the government website providing information to British nationals overseas: