Note that this of relevance only to very few people, and it is probably easier to do applications completely in the UK.
But if you are about to naturalize and you and your British(!) spouse also needs to move somewhere outside the UK (for a job for example), then this is relevant.
Absence rules do not require you to be in the UK when the application is submitted:
Appropriate receiving authority for applications for citizenship that are submitted from overseas:Consideration of whether the requirements for citizenship, as laid down in Schedule 1 of
the 1981 Act, are met is determined in line with that relevant 3 year period. For example,
applicants should:
a. be in the UK at the beginning of the period of 3 years ending with the date of the
application; and
b. not to be absent from the UK for more than 270 days in that 3 year period; and
c. not to be absent from the UK for more than 90 days in the period of 12 months
ending with the date of the application; and
d. not, on the date of the application, be subject under the immigration laws to any
restriction on the period of stay in the UK; and
e. not, at any time in the period of 3 years ending with the date of the application, be in
the UK in breach of the immigration laws.
There is also a section on who can do citizenship ceremonies outside of the UK.Applications for citizenship that are submitted from overseas should be sent to the
appropriate receiving authority as specified in the British Nationality (General)
(Amendment) Regulations 2012. In accordance with these regulations, applications should
be sent to:
i. the Home Secretary if, on the date of receipt, the applicant is in Great Britain or
Northern Ireland;
ii. the Lieutenant-Governor if, on the date of receipt, the applicant is in any of the
Islands (i.e. the Channel Islands or the Isle of Man);
iii. the appropriate Governor if, on the date of receipt, the applicant is in a British
overseas territory;
iv. if the applicant is in Hong Kong, the "receiving authority" is any consular officer,
established officer in the Diplomatic Service or any person authorised to accept
nationality applications;
v. if the applicant is elsewhere, including in a Commonwealth country, to the Secretary
of State at the Home Office.
Applications that are sent to the wrong receiving authority will be deemed invalid.