I copied this from the IND website:
"Naturalisation
Foreign nationals may apply for certificates of naturalisation. This also applies to Commonwealth citizens and Irish citizens. Applicants must:
* have lived legally in the United Kingdom for five years (the last year should have been free of any time limit);
* be 18 or over;
* not be of unsound mind;
* be of good character;
* have sufficient knowledge of English, Welsh or Scottish Gaelic (depending on their age and physical and mental condition); and
* stay closely connected with the United Kingdom.
The wife or husband of a British citizen may apply for naturalisation after living in the United Kingdom legally for three years. The applicant's stay must not be subject to any time limit on the date of application; the applicant must be 18 or over and not of unsound mind, but does not have to meet the requirements to have sufficient knowledge of the language and to intend to live in the United Kingdom once naturalised. He or she must be of good character, however.
To meet these residence requirements, you must not be absent from the United Kingdom for more than 450 days over the 5-year period, or for more than 270 days over the 3-year period. In each case, you must not be absent for more than 90 days in the last year of the period."
Though that last part was kind of important.
Also, the husband or wife of a British citizen may apply for dual citizenship on behalf of any children who meet the residency requirements as long as application is made before their 18th birthday. After 18, the child must apply on their own basis.
NB Any emphases in the IND text was added by me.
