Here is a quote from an article called “British Politeness and the Progress of Western Manners: An Eighteenth-Century Enigma” by Paul Langford. It’s eighteenth century but it speaks mostly of visitors to Great Britain after the peace treaty of Amiens, so after 1802!
It was easy to fall foul with of a system which like the British Constitution itself was unwritten yet intimidatingly authoritative. One subject that features frequently in the accounts of visitors to London was the seemingly trivial yet evidently perplexing matter of door-knocking, something that did not feature in conduct manuals. Servants were trained to respond to a hierarchy that extended from the single and almost silent touch of a milkman to the five deafening onslaughts of a duke or his footman. Foreigners initially adopted a hesitant, degerential approach which brought humiliation at the hands of servants. In due course the practise found its way to other cities, reminding visitors that learning to knock like a gentleman was an essential art in Britain.