What should you do?
If you are reimbursed for qualifying home-office expenditure by your employer, the good news is that no action is required in order to qualify for tax and National Insurance relief, as it should not appear on your Form P11Ds for 2020/21, reporting benefits and expenses. The same should also apply for 2021/22.
You should consider whether you qualify for a standard £6 per week for the whole of 2020/21 and make a claim where appropriate.
If you face extended periods of working from home in 2021/22 and no new measures are announced, you should keep records of this time and consider making a claim in due course.