http://en.wikipedia.org/wiki/British_citizenship

The British Nationality Act 1948 established the status of Citizen of the United Kingdom and Colonies (CUKC), the national citizenship of the United Kingdom and those places that were still British colonies on 1 January 1949, when the 1948 Act came into force. However, until the early 1960s there was little difference, if any, in United Kingdom law between the rights of CUKCs and other British subjects, all of whom had the right at any time to enter and live in the United Kingdom.

http://en.wikipedia.org/wiki/Citizen_of_the_United_Kingdom_and_Colonies

From 1 January 1949, every person who was a British subject by virtue of a connection with the United Kingdom or one of her crown colonies became a Citizen of the United Kingdom and Colonies.

However, UK & Colonies citizens, in common with citizens of other Commonwealth countries, also retained the status of British subject. From 1949, the status of British subject was also known by the term Commonwealth citizen, and included any person who was:

* a citizen of the United Kingdom and Colonies;
* a citizen of any other Commonwealth country; and
* one of a limited number of "British subjects without citizenship".

http://en.wikipedia.org/wiki/History_of_British_nationality_law

Acquisition of Citizenship of the UK & Colonies

Under the 1948 Act, CUKC status was acquired by:

* birth in the UK or a colony (exceptions for children of 'enemy aliens' and diplomats). The immigration status of the parents was irrelevant.
* naturalisation or registration in the UK or a colony or protectorate
* legitimate descent from a CUKC father for children born elsewhere. Only the first generation acquired British nationality automatically. Second and subsequent generations could do so only if born outside the Commonwealth (or Ireland) and registered within 12 months of birth or if the father was in Crown Service.
* incorporation of territory (no persons ever acquired CUKC this way from 1949)
* declaration
* marriage

Provisions for acquisition of CUKC by adoption were not included in the 1948 Act itself but were added soon after.

關於英國國籍在雙重國籍的部分:

Dual nationality and dual citizenship

Since the British Nationality Act of 1948, there is in general no restriction, in United Kingdom law, on a British national being a citizen of another country as well. So, if a British national acquires another nationality, they will not automatically lose British nationality. Similarly, a person does not need to give up any other nationality when they become British.