If you are employed or receive a pension and earn interest over your personal allowance but under £10,000 (You need to register for Self-Assessment if your income from savings and investments is over £10,000 in the financial year) and are employed or receive a pension, HMRC will change your tax code so you pay the tax due 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: https://www.gov.uk/apply-tax-free-interest-on-savings