By an accident of history (Caxton - whose role in standardization was pivotal, because printing had the power to form the basis of a standard - came from Birmingham I think, and many of the major forces [throne, church, law courts, 'the corridors of power'] were in the south) BBC English/Oxford English/the Queen's English - the formal 'standard' - is based on southern speech. However, the BBC stopped being exclusively southern-speaking many years ago, and regional accents are now perfectly acceptable. If one person says /ba:θ/ (as I do) and another says [bæθ] (as my father - a northerner - did) there's no big problem; it's just made me a bit of a chameleon - I regularly pronounce 'garage' either /'gæra:ʒ/ or /'gæra:ʤ/ or /'gærɪʤ/ or /gə'ra:ʒ/ depending on who I'm talking to.
But Chaucer, writing in the 15th century, used 'comic northerners' in one of the Canterbury Tales, and ridiculed their vowels. And in the 6 centuries since then there has been a tendency (sometimes strong, sometimes very weak - but always there) for people in formal contexts to favour a southern pronunciation. (This is resisted strongly in the north, of course.)
b