Income Tax - Double taxation UK and Japan

I am a British national with permanent residency visa status in Japan.

In brief, I am thinking of buying a 'buy-to let' property in the UK.

I know that I will have to pay income tax in the UK but I want to know if I'll also have to pay income tax in Japan.

Also, if I do have to pay income tax here in Japan, how much will I have to pay?

I believe that if you have been living in Japan (under work or permanent resident status) for longer than five years in the past ten years, you are considered a "(permanent) resident in Japan" in terms of taxation, and you need to declare all your worldwide income, in Japan. Whether you can claim something back with the UK authorities due to double taxation is an issue of any tax agreement between the UK and Japan.

I find the above information in the annual income tax filing guide that my spouse receives, in English, each year, but that document does not seem to be available in full online. This link sets out some similar information:
http://japantax.org/?p=3117
https://injapan.gaijinpot.com/uncategorized/2010/08/10/taxes-in-japan/

Get real advice from a professional, not a forum. Double taxation is an important issue and you can be treated as tax resident spending 183 days in one year in a country, or having a house or substantial assets. So living in Japan would make you tax resident, and having a house/apartment in the UK might also make you tax resident there.

Thank you for your comments.
I agree that income tax is an important issue and I need professional advice.
What I really need to know is who/which organization should I ask?

tax accountants licensed by each governments.
UK accountants don't know the Japan law. and Japan accounts don't know the UK law. you have to ask the same question independently.
generally speaking, about the UK income,
firstly you have to follow the UK tax law as a non-resident person. then, you have to follow the Japan tax law as a resident person.

Yes - seek advice in the UK and Japan. It is not simply about being a non-UK resident as the classification is "tax resident" - you don't have to be living there.

In my situation, I am tax resident in two countries currently. Simply owning a car in another country qualifies me for that according to the tax rules that apply.