As the property being let is in the UK and under article 6 of the UK/Japan double taxation agreement, the income from this property is taxable in the United Kingdom.
UK/Japan Double Taxation Convention
You are a non resident landlord and should be completing Self Aassessment tax returns each year. You can see more information here:
Check if you need to send a Self Assessment tax return
You may also have to pay tax on the property in Japan. If that is the case, you claim a tax credit for the tax paid in the UK and thus avoid double taxation.
HMRC is currently running a let property campaign, allowing disclosure relating to the past tax years:
Let Property Campaign: your guide to making a disclosure
There are no Income Tax implications arising from the transfer of money to the UK.