He will be marked down as a firm refusal, which will lead to follow ups from more experienced people working on the census.
The person on the video was just a census collector who is supposed to be a friendly face who reminds and offers help to people who haven't filled theirs in.
She did very quickly mention the £1000 fine which is supposed to be a last resort, but she probably judged the situation correctly that there was no convincing this guy.
The funny thing is that people who are marked as firm refusals will be chased more aggressively than people who just procrastinate and end up not doing it. He is much more likely to get a fine now if he keeps refusing.