I don’t think there is anything wrong with door knocking. just knock and stand back 2m
Personally I think people would be more understanding during this crisis than in normal times, everyone knows it s a hard time for business and we all need to stick together to get through this. If. Someone like a window cleaner knocked on my door, was decent looking, polite and was honest and said they were struggling during this time and were looking for extra customers I would go out of my way to help.
People now have a Better understanding of COVID , it’s not like when it first hit and everyone was scared they know that there is little risk when people keep their distance.