Courses for learning RP:
https://receivedpronunciation.thinkific.com/collections
Online forum
http://forum.receivedpronunciation.com/
Famous speakers of RP: http://forum.receivedpronunciation.com/viewtopic.php?f=3&t=3
https://www.pinterest.com/mypronunciation/
Facebook groups:
https://www.facebook.com/groups/mypronunciation
https://www.facebook.com/groups/rpenglish
https://www.facebook.com/mypronunciation
Wikipedia materials:
https://en.wikipedia.org/wiki/International_Phonetic_Alphabet
https://en.wikipedia.org/wiki/Received_Pronunciation
https://en.wikipedia.org/wiki/General_American_English
https://en.wikipedia.org/wiki/Mid-Atlantic_accent
https://en.wikipedia.org/wiki/IPA_vowel_chart_with_audio
https://en.wikipedia.org/wiki/IPA_pulmonic_consonant_chart_with_audio
https://en.wikipedia.org/wiki/Stress_(linguistics)
https://en.wikipedia.org/wiki/Voice_(phonetics)
https://en.wikipedia.org/wiki/Glottal_stop
https://en.wikipedia.org/wiki/Glottalization
https://en.wikipedia.org/wiki/Airstream_mechanism
https://en.wikipedia.org/wiki/Intonation_(linguistics)
https://en.wikipedia.org/wiki/Tone_(linguistics)
https://en.wikipedia.org/wiki/Stress_(linguistics)
https://en.wikipedia.org/wiki/Vocal_resonation
https://en.wikipedia.org/wiki/Vocal_register
https://en.wikipedia.org/wiki/Vocal_cords
https://en.wikipedia.org/wiki/Elocution
https://en.wikipedia.org/wiki/Accent_reduction
https://en.wikipedia.org/wiki/English_phonology
https://en.wikipedia.org/wiki/Phonetics
https://en.wikipedia.org/wiki/Speech
https://en.wikipedia.org/wiki/Syllable
https://en.wikipedia.org/wiki/Linking_and_intrusive_R
https://en.wikipedia.org/wiki/Diphthong
https://en.wikipedia.org/wiki/Monophthong
https://en.wikipedia.org/wiki/Semivowel
https://en.wikipedia.org/wiki/Vowel_length
https://en.wikipedia.org/wiki/Velarization
https://en.wikipedia.org/wiki/Muscle_memory
https://en.wikipedia.org/wiki/Spaced_repetition
https://en.wikipedia.org/wiki/Behaviour_therapy
https://en.wikipedia.org/wiki/Feedback
https://en.wikipedia.org/wiki/Head_voice
https://en.wikipedia.org/wiki/Chest_voice
https://en.wikipedia.org/wiki/Isochrony
https://en.wikipedia.org/wiki/Prosody_(linguistics)
https://en.wikipedia.org/wiki/Manner_of_articulation
https://en.wikipedia.org/wiki/Nasalization
Rhotic vs Non-rhotic accents
https://www.thoughtco.com/rhoticity-speech-4065992
https://www.voicescienceworks.org/breath.html
What is Received Pronunciation? (Note that the definition can vary slightly.)
https://www.collinsdictionary.com/dictionary/english/received-pronunciation
https://dictionary.cambridge.org/dictionary/english/received-pronunciation
https://www.thefreedictionary.com/Received+Pronunciation
The Mid-Atlantic Accent (American equivalent of RP)
https://www.snopes.com/fact-check/mid-atlantic-accent/
https://www.urbandictionary.com/define.php?term=Mid-Atlantic%20accent
BBC English
http://www.bbc.co.uk/learningenglish/english/features/pronunciation/longvowel1
Received Pronunciation in the media:
https://www.bbc.com/future/article/20180307-what-does-your-accent-say-about-you
https://www.independent.co.uk/voices/accent-discrimination-work-job-prejudice-b1399561.html
https://www.dailymail.co.uk/tvshowbiz/article-8852721/Lily-Collins-wishes-English-accent.html
https://www.britishcouncil.org/voices-magazine/uk-accents-not-what-you-say-how-you-say-it
https://abagond.wordpress.com/2008/06/18/received-pronunciation/
http://www.classicbond.com/forum/viewtopic.php?t=2405
Miscellaneous links
https://teflpedia.com/Received_Pronunciation
https://wiki2.org/en/Conservative_Received_Pronunciation
https://www.bl.uk/british-accents-and-dialects/articles/received-pronunciation
https://airodyssey.net/reference/inflight/#doors