John Wells, at his phonetic blog, has a post offering a professional analysis of just how an American voice teacher went wrong in a video clip in which she tries to teach the British “short o” vowel. I particularly like this paragraph:
Her happY vowel (at the end of coffee) is much too open. It approaches ɛ or perhaps more precisely [ɛ̝̈], which in England is highly marked both socially and regionally. Socially, it belongs in a variety of U-RP which is probably now entirely obsolete, a subvariety of what Cruttenden calls “Refined RP”. Alternatively, geographically it is associated with (the working-class accent of) central Northern places such as Leeds. No actor should use this kind of happY vowel for “British” unless playing an upper-class character in a play set a hundred years ago or more.
His conclusion: “Tracy’s version of BrE represents an impossible mixture of different social classes and different geographical locations. Bits … of it are Scottish, bits of it are northern English, bits are RP/southern. Some of it is caricature-upper-class, some of it is working-class. Nobody, but nobody, talks like that in real life.” You can see the video at that link; here‘s a hilarious parody by a Brit explaining how to pronounce the American short o. (Both links courtesy of Dave Wilton at Wordorigins.org.)