Under current British law, citizenship decisions rest with the Home Office, applying criteria established by Parliament through the British Nationality Act 1981. Naturalization requires residence, language proficiency, good character, and an intention to remain in the UK. A foreign monarch seeking British citizenship would theoretically follow these standard procedures, with no provision for public referendum or direct voting. However, in this special scenario for a monarch where people's votes determine such citizenship, we must imagine a reformed system. Perhaps Parliament, responding to exceptional circumstances—such as a foreign monarch seeking asylum or a proposed constitutional union between nations—might legislate for a special referendum. This would represent an extraordinary democratic innovation, giving citizens direct say over a matter typically reserved for administrative discretion.

Should people vote to grant citizenship, the Home Office would presumably issue naturalization papers, and the foreign monarch would formally become British while retaining their foreign throne—creating an unusual dual status. Rejection would send a powerful message about popular will, potentially straining diplomatic relations but asserting democratic control over national membership.