If you are an employee and were told by an employer to work from home during from April 2020, even just for one day, you could make a claim for a working from home tax refund online via HMRC directly to alter your tax code.
The tax code alteration would reduce your tax bill by either £62 or £125 for each of the 2020 and 2021 tax years. The difference is based on your highest tax bracket, either 20 percent or 40 percent of an expense allowance of £6 a week for each tax year.
The new facility was introduced when the pandemic required people to work from home providing the employer allowed it. Self employed people can claim it too but it would go through their usual home working expenses.
So, if you're a 40 percent taxpayer and want £10 off your monthly tax bill (or £5 off if you're a 20 percent taxpayer) head over to https://www.gov.uk/tax-relief-for-employees/working-at-home.
You just log in with your government gateway ID and password (if you don't have these you can register with your NI number and either a payslip, P60 or passport).
The HMRC site will let you know if you are eligible after asking a few questions and then your tax code will be updated to reflect the refund due to you.