valuable

guessing

I think the phrase does not refer to the person's rank or work ability or achievements. I think it is more general, more about the personality, the maturity of the person. The phrase 「レベルの低い人間」 usually refers to an immature, more "primitive" person that does not conform to the normal social and/or cultural expectations of society, or those around him. For example, someone that would not dress properly, whose hair is greasy, who eats messy, or takes pleasure in low-intelligence entertainment. Of course, the threshold varies by the onlooking person, and the given environment.
I think the office lady saying "I like (more) sophisticated people/men" would fit the context.

...If they like British English, "Diamond Geezer" if they are into Cockney, or "high roller" or anything ending with "daddy" if Americanized.

I wouldn't disagree with the other suggestions per se, but stylistically they strike me as rather stiff and non conversational. Which is why I suggested those more colloquial terms. For the opposite - so many to choose from depending on one's standards - Neanderthal, mouth breather, red neck, knuckle dragger, caveman . . .