UK citizenship law is based on a mixture of ius soli and ius sanguinis and has been largely shaped by the UK’s colonial history. Presently, one may acquire UK citizenship at birth, if born in the UK to at least one parent who is a British citizen or settled in the UK. One may also acquire citizenship by means of descent if born outside of the UK to a British citizen who holds British citizenship by means other than by descent.
In terms of acquiring British nationality after birth, this is possible through naturalisation. In order to satisfy the requirements for naturalisation, one must show knowledge of life in the UK and of the English language, show prove of lawful residence in the UK of at least 5 years (with absences from the UK of no more than 450 days over this period and specifically not more than 90 days in the final year) before the date of application, and the final year must be spent free from immigration restrictions. Applicants must intend to live in the UK and must also be of ‘good character’.
Where an applicant is married to a British citizen the acquisition of citizenship by naturalisation is easier since residency period is reduced to three years (with absences from the UK of no more than 270 days over that period and again not more than 90 days in the final year). In addition an applicant only needs to be free from immigration restrictions at the time the naturalisation application and there is no requirement to intend to live in the UK.
With respect to dual citizenship, this is allowed in the UK and any British citizen acquiring second citizenship will not lose their original citizenship. Reciprocally, foreign nationals who acquire British citizenship are not required to renounce their original nationality. British citizens are not required to register or justify their dual citizenship status.