My understanding is that Japanese nationals who receive taxable income from the UK are entitled to the UK Personal Allowance based on their nationality, under UK domestic law as outlined in HMRC Guidance (RDRM10340).
Japan is included in the list of countries whose nationals are eligible the for UK Personal Allowance, which includes:
Argentina, Australia, Azerbaijan, Bangladesh, Belarus, Bolivia, Bosnia and Herzegovina, Botswana, Canada, Côte d’Ivoire (Ivory Coast), Egypt, Gambia, India, Indonesia, Japan, Jordan, Kazakhstan, Korea, Lesotho, Malaysia, Montenegro*, Morocco, New Zealand, Nigeria, Oman, Pakistan, Papua New Guinea, Philippines, Russian Federation, Serbia*, South Africa, Sri Lanka, Sudan, Switzerland, Taiwan, Tajikistan, Thailand, Trinidad and Tobago, Tunisia, Turkey, Turkmenistan, Uganda, Ukraine, Uzbekistan, Venezuela, Vietnam, Federal Republic of Yugoslavia*, Zimbabwe.