Ah, thanks for that. I expect that no-one will be fined unless they fail to complete the census or put obviously nonsense information despite repeated reminders from the census officers.
Which is fair enough, assuming there's protection in place so they don't fine someone who had been in hospital or genuinely incapable of completing the form.
DM did this role 30 years ago with my assistance and in that time, we had to visit every house in person to hand out the forms, collect them some time later, chase up any stragglers and give help if anyone couldn't complete the form. This time it all seemed very easy, done online in a few minutes.