The amount arising before tax is taken off would be the gross figure before any deductions. Enter the tax witheld in the 'Witholding tax and any tax taken off'' box. Then the taxable anount would be the net amount after deductions from the gross figure.
You may be entilted to claim Foreign Tax Credit Relief, please check article of the relevant Double Taxation Agreement before completing this section to make sure you are able to claim Foreign Tax Credit Relief as certain types of income are only taxable in the UK. You can find guidance here:
Relief for Foreign Tax Paid 2022 (HS263) Updated 6 April 2022
You can also see SA106 in the link above for notes of foregn interest reveived.