You need to submit an Income Tax return (for 2023/24) if HMRC send you a Tax Return to complete or, if you know that you have income tax or capital gains tax to pay.
However, HMRC have a list of further circumstances where a Tax Return should be prepared, including:
Your income from self-employment exceeded £1,000;
You are a Partner in a business partnership;
You received over £2,500 in rental income. If it was between £1,000 and £2,500 you should contact HMRC;
You received more than £2,500 in other income such as tips or commission;
You received £10,000 or more before tax from savings/investments;
You made Capital Gains in excess of £6,000;
You have received dividends in excess of £10,000;
Your taxable income exceeds £50,000 and you, or your partner, receive Child Benefit;
Your taxable income exceeds £150,000.