From Wikipedia....
British Citizenship can be acquired in the following ways:
lex soli: By birth in the United Kingdom to a parent who is a British citizen at the time of the birth, or to a parent who is settled in the United Kingdom
lex sanguinis: By birth abroad, which constitutes "by descent" if one of the parents is a British citizen otherwise than by descent (for example by birth, adoption, registration or naturalisation in the United Kingdom). British citizenship by descent is only transferable to one generation down from the parent who is a British citizen otherwise than by descent, if the child is born abroad.
By naturalisation
By registration
By adoption
http://en.wikipedia.org/wiki/British_nationality_law
