At last, this is starting to be discussed in the Japanese government.

The current law states that Japanese cannot have dual nationality, which means that Yuko would need to give up her nationality if she ever got a British passport.
It would also mean that any children of ours would be dual nationality until the age of 22, at which point Japanese law forces them to make a choice.

But then, I saw this bit in the article.

Now, Taro Kono, a Lower House member of the Liberal Democratic Party, the larger of the two-party ruling coalition, is trying to iron out another wrinkle in the law that became apparent in October when it was learned that Tokyo-born Nobel Prize winner Yoichiro Nambu had given up his Japanese nationality to obtain U.S. citizenship.
People like Nambu follow the letter of the law with respect to the Constitution’s Article 14, which requires that Japanese renounce other nationalities by the age of 22 if they wish to keep Japanese citizenship. Yet, according to Kono, there are 600,000 to 700,000 Japanese 22 or older with two nationalities, if not more. In other words, fewer than 10 percent of Japanese with more than one nationality make that choice by the time they turn 22, Kono said.

Now, I knew there were some high profile cases (former Peruvian President Alberto Fujimori for example), but didn’t realise that it was ‘known’ about these other 600-700k people. Effectively, the Japanese authorities are turning a blind eye to it all, making a mockery of the system.