Sole traders and partnerships have to include their accounts figures on self assessment TAX RETURNS and file these by 31st January following the end of the tax year at latest.
A tax year is 6th April to 5th April in the UK, so the tax year 2020/2021 is 6/04/2020 to 5/04/2021, and the Tax Return for this year needs to be filed by 31st January 2022.