Oof! This has got tetchy very quickly.
I agree that a lot of people take Southern pronunciation as the default and assume that anything from North of Watford is a deviation, but that's accent, not dialect. When I worked full time I heard so many students claim not to have an accent, when they absolutely did - they just thought that Southern accents were 'correct'. Even 'Cor blimey' Landan was seen by them as 'posher' than Scouse or Yorkshire in their eyes, which is ridiculous.
I love regional variations, and would be sad to see them go - particularly phrases and sayings from the past.
I think that children should be taught to write in Standard English, but definitely not forced to ditch an accent to speak in RP, or to think that anyone who does use RP has no accent, as that is an accent like any other.