Register marriage at the Japanese embassy

I am a British citizen and my wife is a Japanese national with indefinite leave to remain in the UK. So there are no visa issues.
We need to register our marriage at the Japanese embassy in London but what document do we need to take with us?
The 3 month time limits has almost elapsed.
Any help would be greatly appreciated! Thank you in advance! 🙏🏻

https://www.japan-guide.com/forum/quereadisplay.html?0+90747

This is what I know about registering a non-Japanese law based marriage to the Japanese authorities.

Let your Japanese wife read #2 of the following guide in Japanese by the embassy.
http://www.uk.emb-japan.go.jp/itpr_ja/index_000051.html

Among the necessary documents are
an official transcript of a marriage certificate
and its translation into Japanese.

To be precise:
her marriage with you
can not be registered at the Japanese embassy
but should be recorded in her family register [koseki].

She needs to know which municipality manages her family register so that she can get its transcript / abstract [koseki-touhon / -shouhon].
(For reference: My family register is managed by a city in whose area my grandparents lived in the past.)

I recommend she also checks the following Q & A
(, skipping what are not related to you or her).
- Ministry of Justice: Civil Affairs Bureau
: Q and A as to family register regarding international marriage, childbirth overseas, etc. ((in Japanese))
http://www.moj.go.jp/MINJI/minji15.html

Thank you in advance!

Personally I take the pose of the palms put together as in this icon
for petition (e.g. "May the new year be a good year."), entreaty ("Oh, please!") or apology, rather than appreciation.

Marriage should be registered at the city hall in Japan as well in England.
The embassy is pure for documentation. They are not interested if you are married or not.

Even when you marry in Japan you can just go to the city hall fill in a document.

OP and the Japanese spouse are already married, so their UK law based marriage needs to be reported to the Japanese authorities after-the-fact, either "through" the embassy/consulate services if they live outside Japan, OR at a city hall in Japan. It is the responsibility of the Japanese national to report changes in their status, such as marriage.