RP refers to the accent not grammar.
"Received Pronunciation (RP) is the accent traditionally regarded as the standard and most prestigious form of spoken British English.[1][2] For over a century, there has been argument over such questions as the definition of RP, whether it is geographically neutral, how many speakers there are, whether sub-varieties exist, how appropriate a choice it is as a standard and how the accent has changed over time.[3] The name itself is controversial. RP is an accent, so the study of RP is concerned only with matters of pronunciation; other areas relevant to the study of language standards such as vocabulary, grammar, and style are not considered."