The antigen fit to fly is for all that requirements for anyone requiring a pre departure test back to the U.K.
Before returning to the U.K. you need to have booked a Day 2 Diagnostic PCR test to be taken on the second full day of returning to the U.K. The reference number of the Day 2 needs to go on your passenger locator form
List of requirements for travellers going back to England shown here. If you’re travelling with under 18 year olds and travelling back to Wales, Scotland or NI check the rules for your country as they have completely different rules and different age brackets.
Lynn