Until you break UK residence, nothing changes
The UK taxes residents on their worldwide income, so a move to Japan only helps once you are non-resident under the Statutory Residence Test. The usual route for movers is working full-time abroad, broadly at least 35 hours a week, with fewer than 91 UK days and no more than 30 UK workdays, which a normal Japanese secondment fits comfortably. Leave part-way through a tax year and split-year treatment can tax you as non-resident from your departure date. It is claimed on the SA109 pages of your Self Assessment return; the P85 does not do this for you.
What stays UK-taxable after you leave
Non-residents still pay UK tax on UK-source income. Rental profits fall under the Non-Resident Landlord Scheme, and gains on UK property must be reported and any tax paid within 60 days of completion, even where nothing is due. Pensions need care: Article 17 of the 2006 UK-Japan treaty gives Japan sole taxing rights over most UK pensions once you are Japan-resident, but Article 4(4) limits the UK relief to amounts actually remitted to Japan while Japan taxes you on the remittance basis, so a pension paid into a UK account and left there can stay UK-taxable. Return to the UK within five years and the temporary non-residence rules can tax gains realised abroad in your year of return, and the residence-based inheritance tax tail can run for up to 10 years after departure.
The Japanese side: a five-year on-ramp
Japan taxes long-term residents on worldwide income at high marginal rates, but not from day one. A foreign national whose aggregate stay is five years or less within the preceding ten is a non-permanent resident: Japan-source income is taxed in full, foreign-source income only to the extent it is paid into Japan or remitted there. National income tax runs from 5% to 45% plus a 2.1% surtax and a flat 10% inhabitant tax, while most listed investment income is taxed separately at a flat 20.315%. Stay beyond five years and worldwide taxation begins, along with potential exposure to Japan's exit tax on financial assets worth JPY 100 million or more. The account structure that keeps UK income unremitted needs setting up before you land, so read our guide to UK tax when moving to Japan before you book flights. Horizon handles the UK side of the corridor on a fixed fee agreed upfront, and a free clarity call at /book will tell you what your departure should look like.