CGB stands for Conspicuous General British. Who uses this variant? According to Cruttenden it is
to be associated with upper-class families, with public schools and with professions which have traditionally recruited from such families, e.g. officers in the navy and in some army regiments (81).It is "commonly considered to be 'posh'" (81).
What are typical phonetic features? Here's a selection:
1. use of the KIT vowel in unstressed word-final position as in 'there's a universit/ɪ/ in our lovel/ɪ/ cit/ɪ/';
2. a very open word-final schwa as in 'wait[ɐ]', 'moth[ɐ]';
3. the ash vowel is frequently diphthongised as in [mɛəd] for 'mad'.
RGB is a hybrid variant mixing GB with a few regional features. Cruttenden concedes that the term should actually be used in its plural form - RGBs. In comparison with CGB, RGB is a cover term for regional variants rather than a marker of class or, in Cruttenden's words: "[...] it is useful to have such a term as RGB to describe the type of speech which is basically GB except for the presence of a few regional characteristics which may well go unnoticed even by other speakers of GB" (81). One of his examples is the vocalisation of dark l, which "passes virtually unnoticed in an otherwise fully GB accent" (82).