https://en.wikipedia.org/wiki/Received_Pronunciation
A partial extract from the Wiki:
Nasals and liquids (/m/, /n/, /ŋ/, /r/, /l/) may be syllabic in unstressed syllables.[62] The consonant /r/ in RP is generally a postalveolar approximant,[62] which would normally be expressed with the sign [ɹ] in the International Phonetic Alphabet, but the sign /r/ is nonetheless traditionally used for RP in most of the literature on the topic.
Voiceless plosives (/p/, /t/, /k/, /tʃ/) are aspirated at the beginning of a syllable, unless a completely unstressed vowel follows. (For example, the /p/ is aspirated in "impasse", with primary stress on "-passe", but not "compass", where "-pass" has no stress.) Aspiration does not occur when /s/ precedes in the same syllable, as in "spot" or "stop". When a sonorant /l/, /r/, /w/, or /j/ follows, this aspiration is indicated by partial devoicing of the sonorant.[63] /r/ is a fricative when devoiced.[62]