link:
1. The penis.
2. An idiot, an objectionable person.
link:
5. (British, slang) The penis.
6. (slang) (pejorative) A contemptible person.
So if he was saying "knob" William Troubshaw could have been calling Higgins a contemptible or more likely an objectionable person. I'm doubting that he meant penis, especially since his wife was right there and she was actually in between William and Higgins. He might have been a jerk to Higgins, but I don't think he was the kind of man to hurl a simple penis insult over his wife's head.


But there's also nob:
- noun Chiefly British Slang.
a person of wealth or social importance.
He could have used this facetiously, meaning Higgins was below their social standing or class. Or since they went to school together, William Troubshaw could have been highlighting the fact that Higgins had not quite risen to the elite social or economic standing that William had. Or he simply could have used nob correctly, as they both attended "one of the most prestigious public schools in all of England" and saved his insult for his next line: "You're looking well. Well-fed that is."
But then there's also this definition for nob:
an elegantly dressed man (often with affected manners)
A person in a superior position in life; a nobleman. [Slang]
I had to look up "affected manners"

So I'm thinking it was the word nob he used, not knob. The subtitles can be wrong sometimes, but nob is what they choose to use here.