Non-U and Plebeian

Non-U adjective - Not characteristic of the upper classes especially in language use.

Plebeian and non-U are semantically related In some cases you can replace term "Plebeian" with "Non-U", this terms are similar.

Plebeian adjective - Belonging to the class of people of low social or economic rank.

Non-U and plebeian are semantically related (British) in uncultivated topic. You can use "Non-U" instead an adjective "Plebeian". informal substitute