Hi,
Your banks and building societies will automatically inform HMRC of any untaxed interest received after the end of the tax year. If you earn interest over your allowance but under £10,000 and are employed or receive a pension, HMRC will change your tax code so you pay the automatically.
The code for the current year will be based on how much interest you had in the previous year.
Further information can be found here:
Tax on savings interest
Thank you