There are basically three methods through which you can acquire a UK citizenship:
1. Through naturalization
To be a British citizen through naturalization the home secretary must deem you fit by considering various set rules and laws. Usually you must be married, in a civil relationship or partnership with a British. You must also be of good character, must have lived in the UK for three years, speak English well and be able to survive in the UK.
Those not in a civil relationship or partnership must have five years of legally living in the UK.
2. Through registration
Registration is a much simpler method but with more restrictions than naturalization. British nationals with over five years of residence in the UK are eligible for registration. Children born and raised up until they are ten are also eligible.
3. Though adoption
A child adopted by a UK citizen will automatically acquire citizenship if the adoption was legally carried out without compromising the laws of the land.