@MatthewDave is quite right in saying that a sophisticated person's language would be distinguished by lack of 'lower-class' colloquialisms. Add to that impeccable grammar, and a rich vocabulary.
By rich vocabulary I do not mean random use of fancy words. Instead, I mean words with a narrower meaning, that fit the given situation with greater precision. For example, above, I could have said "good grammar" or "perfect grammar". "Impeccable" has a narrower meaning than either, so it's less likely for a person to have seen it. Using the word with ease implies one is sufficiently well-read to have the word not only in one's passive vocabulary (understanding what it means), but also in one's active vocabulary (using it correctly). And being well-read remains to this day a mark of upper classes. It was even more so in the past.
Inserting archaic words that are no longer in use is not a mark of "posh" English. Instead, it is the shorthand Holywood use to signify something is happening in the past, or that a text being read is old (a trope known as Ye Olde Butcherede Englishe). Moreover, Holywood usually use older words incorrectly, disregarding period grammar and often muddling what the words actually meant. (For instance, 'thou' is often used in movies as a politer form of 'you'. In truth, the opposite is the case. 'Thou' was second-person singular, while 'you' was second-person plural. Etiquette dictated that social superiors, and then equals too, be addressed in the plural, until finally the plural supplanted the singular entirely. A similar form of address still exists in French, Spanish and Russian, among other languages. For more information, see T-V distinction.)
Another mark of good education, and therefore of higher class, is maintaining the same register throughout one's conversation (or changing registers deliberately for a humorous effect). A register is the variety of language used for a particular purpose or in a particular social setting. For example, language might be more formal, or it might be less formal. In your example, however, "quite generous" and "nice bonus" belong to different registers.