Lizbethann55. You never know when circumstances will change, so it should be filled in on the 21st, regardless of Covid rules.
My DD and GD have unexpectedly had to move in with me, due to a hiccup in their house move. They will probably still be here on census day, but we don’t know yet. Although they were both in my ‘bubble’ anyway the rules would have permitted us to share accommodation.
Also, if the form was filled in early, then someone died before the due date, I’m sure that the bereaved wouldn’t even think about notifying the census people of the alteration.
Why do people think that it’s ok to change the rules to suit themselves?