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Hi thank you for your message, so if his parents were married at the time of his birth then he would automatically be a British citizen but because they were not he can still be British but he needs to apply. To do this he must show that his father was a British citizen when your partner was born and he must have been born or adopted in the UK as in this case. He will need to pay: £80 to attend a citizenship ceremony and £19.20 to have his biometric information (fingerprints and a photo) taken. He can apply online here: https://visas-immigration.service.gov.uk/product/nationality-ukf?_ga=2.246644996.1801767782.1578775261-1268263661.1577054567
There is guidance here: https://www.gov.uk/government/publications/form-ukf-guidance
He will be asked to make an appointment at a UK Visa and Citizenship Application Services (UKVCAS) service point to provide his biometric information (fingerprints and a photo). He does not need to send his documents anywhere. He can either:
upload copies into the online servicehave them scanned at his UKVCAS appointment
He can also get help filling in the form here:
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