**Use the actual Japanese income tax paid—about 3% in your case**, provided that is your final tax amount and the earnings are also taxable in the UK. Your credit is capped at the UK tax attributable to those same earnings. [UK–Japan treaty, Article 23](https://www.mofa.go.jp/policy/treaty/submit/session164/agree9-1.pdf).

“Allowed by treaty” means a **limit on the foreign tax eligible for credit**, not an amount you can claim regardless of what you paid. Where a treaty imposes a cap, the credit is the smallest of:

- The foreign tax actually paid.
- The foreign tax permitted under the treaty.
- The UK tax due on the same income.

HMRC’s [HS263 guidance](https://www.gov.uk/government/publications/calculating-foreign-tax-credit-relief-on-income-hs263-self-assessment-helpsheet/relief-for-foreign-tax-paid-2023-hs263) illustrates how that cap works.

Also, **the 10% rate does not apply to your salary**. Employment income falls under Article 14 of the UK–Japan treaty, which permits Japan to tax work performed there and sets no percentage cap. Assuming your employer was resident in Japan, working there for fewer than 183 days does not itself exempt your salary from Japanese tax. [Treaty, Article 14](https://www.mofa.go.jp/policy/treaty/submit/session164/agree9-1.pdf).

For example, if your gross Japanese earnings were £10,000, your final Japanese income tax was £300, and the UK tax attributable to those earnings was £2,000, your credit would be **£300**.