Good news! HMRC have just announced that the 31 January 2021 tax return submission deadline is now extended to 28 February 2021, without getting a late filing penalty. However the Self Assessment tax bill is still due by 31 January 2021. https://www.gov.uk/self-assessment-tax-returns/deadlines 31 January deadline extended!  

The deadline for 2019/20 tax return submission and tax payment is 31 January 2021.