The government guidance on Working Safely for shops, branches, and close contact services has been updated to confirm that from 11 January, people in England who receive a positive lateral flow test will be required to self-isolate immediately but won’t need to take a confirmatory PCR test.
You can find the full guidance here: https://www.gov.uk/guidance/working-safely-during-covid-19/shops-branches-and-close-contact-services