If you are settled (have Indefinite Leave to Remain – ILR or, in the case of European (EU) nationals, permanent residence) in the UK then you can stay here without any time limit, whatever your nationality. You do not have to become a British Citizen. However, this is something many people who have made the UK their permanent home do wish to consider.
There are now three main ways to become a British Citizen. The first is to be born one. The second is to be registered as one and the third is to be naturalised as one. In addition there are circumstances in which a person who was not born a British Citizen automatically becomes one, without the need to be registered or naturalised. For example, this can happen where a child is adopted by British Citizen parents.
Whether or not you were born a British Citizen depends on a combination of where and when you were born and the nationality of your parents. British nationality law is one of the most complicated in the world, in part because of Britain’s history and historical relationship with other countries in the world. In some cases it will be necessary to go back several generations to identify whether you are a British Citizen or are entitled to British Citizenship.