https://en.wikipedia.org/wiki/Received_Pronunciation
https://www.britannica.com/topic/Received-Pronunciation
Learn the modern British RP accent, syllabus for British Received Pronunciation training
https://www.engvid.com/the-rp-english-accent/
https://www.wikihow.com/Speak-the-RP-English-Accent
https://www.actorsdialectcoach.com/post/british-rp