From 1 January 2021, you will no longer be able to purchase items in store in Great Britain under the VAT Retail Export Scheme. For goods purchased before this date, you can still make a claim under the scheme on departure from the UK or EU, in line with the current rules, which allow three months from the month of purchase to claim.
You will still be able to purchase items VAT free from Great Britain if the retailer sends them direct to your address outside the UK.