Thanks for your question. I will try to help.
I would firstly check with a Japanese lawyer that Japan permits dual citizenship, otherwise she may lose her Japanese citizenship if she naturalises as a UK citizen.
She will need to meet the requirements on the following page in order to be eligible to apply for naturalisation:-
In particular you should note the residential requirement as to any time she has spent out of the country in the last three years before her application I submitted and make sure that you have not spent over the amount stipulated. If she has spent more than the time stipulated outside of the UK then she should not apply because it will be rejected.
You will also see that she must have passed the “knowledge of English” and “life in the UK test” and again there are links on the above page giving you information on how she can do this, if she has not already done so.
You will have to apply by completing the Home Office’s Form AN. Here is the link to the page with the form and guidance notes on completing it and what documents you will need to submit with your application:-
The applciaiton fee to apply is £1236.
Once she has applied, if she is successful then she will be sent her Certificate of Naturalisation. She would then attend a citizenship ceremony and thereafter she may apply for a passport.
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