Notes on Individual Rules under the Japan-U.K. Social Security Agreement

In case that you work as an employee on board a sea-going vessel flying the flag of either country, if you would otherwise be compulsorily covered by the pension systems of both countries, you will be subject only to the system of the country in which you are ordinarily reside.
The procedure to apply for your Certificate of Coverage under this rule is the same as it is for general employees.

If you need to continue to work as a detached worker (or a self-employed person) in the other country for more than 5 years because of unforeseen circumstances, you may be granted extension of the period of exemption for up to 3 years. In this case, you continue to remain subject only to the system in your original country where you are detached from. If your application for the extension is not granted, you will be covered only by the system of the country in which you work during the period exceeding the initial 5 years.

Under the Japan-U.K. Agreement, there is not any special rule about accompanying spouse and children.

Provision on voluntary enrollment under the National Pension Law (excluding the voluntary enrollment provision only for the people aged 60 or over) is not be applied to any person who is working in Japan and subject to the U.K. pension laws regarding the liability for contribution payments, till his/her expected detachment period has elapsed.
This means, if you are under age 60, and you are temporarily sent from U.K. to work in Japan and exempt from coverage under the Japanese National Pension Law, you can not voluntarily enroll in the Japanese National Pension system.