In reply to Yanis Nayu:
If I understand correctly, and I may not, but this is how it worked when I went over in August, the LFT you do over there in the 48 hours before returning just generates a certificate that you need to show on leaving (I went by ferry and the ferry people looked at it). To complete the passenger locator form, you need to enter the code of the PCR test you have booked to do after having returned. I had to do it on day 2 and day 8 of getting back, but I think it's different now.
So for the journey back you need the certificate from the LFT test done over there, plus the Passenger locator form, to complete which you need to have pre-booked whatever test(s) the UK requires you to do after returning. You do the LFT test, and complete the locator form in the 48 hours before the return journey.
Post edited at 21:13