Apply for British citizenship if you're married to, or in a civil partnership with, a British citizen - form AN, who's eligible, fees and how to apply. If you meet all the requirements, you can apply for a spouse visa.This allows 2.5 years leave in the UK where you must reside with the UK sponsor. If you are a US citizen wishing to settle in the UK with your British partner, you will need to apply for a UK spouse visa. Marriage to a British citizen does not automatically grant a non-UK spouse British citizenship. Applicants must have lived in the UK with their partner, who is a British citizen or has settled status, for a continuous 60 months. Age Requirement: Both the applicant and their British partner must be at least 18 years old to apply for a spouse visa. 4.3 Naturalisation case papers. A completed application form for a registration of British nationality from 1966. You can apply to naturalise as a British citizen if you are married to a British Citizen and you have Indefinite Leave to Remain (ILR) in the UK.