More than 2.7 million claimed at least one Self-Employment Income Support Scheme (SEISS) payment up to 5 April 2021. These grants are taxable and you should declare them on your 2020 to 2021 tax return before the deadline on 31 January 2022.
https://www.nibusinessinfo.co.uk/content/dont-forget-declare-covid-19-grants-your-tax-return