1st I believe but atm you still need to do the locator form... unsure why as they know where you are via passport and to do it coming back to UK is just stupid to me... surely they shoukd try locate you before you depart not when you are going back as guess what... your going back to UK
Do it now I'm on my way to do mine. Immigration officer told me when I went last to come back before the 25th as mine runs out April the 5th but extensions stop on the 25th :) never been asked for a document to say I can't travel either
Did you take pcr or just apply for a test... if just apply to have a test there is a huge back log... loads of people are in the same position... just be patient