There is a Bible study for internationals in the City of London, as well as a Parents' and Toddlers' group meeting nearby.

Bible studies for international students and others take place at St. Helen's, Bishopsgate.  When numbers attending allow, there is a table dedicated to those wanting to study the Bible in Japanese.  Check out their video:  

A Parents' and Toddlers' group is also held nearby, although this has been suspended temporarily due to the pandemic (there is also a Japanese website).