William Thomas Brown was living at 5 Hadley Street St Pancras in 1920:
https://www.ancestry.co.uk/imageview...2&pId=76854026
With him were father William George, mother Hilda and - too young to vote, but almost certainly under the same roof - brother Gordon Reginald. Hilda was born in 1868 and Gordon in 1902.
William Thomas married late in 1921, but the other three did not move for several years. (and WTB continued to live in the same street)
The Coles family were at the same address.
Using the address search on FMP gives only one family at no 5 and that is the Coles. Looking at Hildas living with Gordons doesn't throw up anything useful.
The logical conclusion seems to be that either no form was delivered, or the entire family was away and just escaped the census. Any bright ideas?