Hi Andre Higgs,
Article 6 of the UK / Japan double taxation agreement, which can be found at:
UK/JAPAN DOUBLE TAXATION CONVENTION SIGNED 2 FEBRUARY 2006, AS AMENDED BY A PROTOCOL SIGNED ON 17 DECEMBER 2013
states that "Income derived by a resident of a Contracting State from immovable property (including income from agriculture or forestry) situated in the other Contracting State may be taxed in that other Contracting State".
This means that if you are resident in the UK and have income from property located in Japan, you are taxable in Japan.
If you remit this income to the UK, you will need to declare it on your tax return, but, you can claim a foreign tax credit of up to 100% of tax deducted in Japan.
Have a look at the guidance on the remittance basis at:
RDRM30000 - Remittance Basis: contents