Research shows that young peoples' accents tend to be "levelling" in the south of England: older people from, e.g. Southampton, Reading, Watford, have more distinctive accents than the young.
Standard English is taken to refer to the set of grammatical rules which are used in written English and perceived to be the "standard" language. However, despite levelling, and the agreement about the "standard", some non-standard forms persist. Often these have geographical associations. One reason for the persistence of non-standard forms is believed to be that language has a social identity function: this is most evident in young people pick up things not from parents but from each other, e.g. "He was like 'what are you doing?'" as a means of reinforcing relationships. Another reason is the migration of people around the country, especially in the case of "new towns".
Non standard dialects and accents tend to be local, while Standard English is not (the written form gives few, if any, clues to the geographical origin of the writer). Trudgill (p30,32) has a graphical representation of dialect/accent variation, with the base of the triangle (x-axis) representing the spread of varieties. Moving up the social classes (the y axis), the peak of the triangle has "RP": i.e. regardless of where you come from, a higher social class is associated with standard English/RP. Evidence today suggests that the base of the triangle is shrinking (i.e. there are fewer varieties).
In the 18th century, the industrial revolution led to great expansion in some towns in the north of England (Manchester, Liverpool, etc.). This resulted in massive migration of people to these areas, which are classified as "old" new towns, as compared to Milton Keynes, which is a "new" new town. By studying the accents of people living in Milton Keynes, some insight may be gained into the way that dialects and accents form in a new town.
Recordings of old people in Milton Keynes now show that they retain accent features of the areas where they grew up. But children living there sound different, and appear to exhibit levelling of accent and dialect; the change has taken place within the space of one generation.
(See handout) Analysis of the speech of children in Milton Keynes shows that the characteristic phonetic patterns of their speech does not correlate very strongly with their care-givers.
Some of the features of Milton Keynes dialect are similar to those found in London (e.g. "bruvver" for "brother", glottal-stops). But MK is not the same as "cockney" - for example, the children say "about", not "abaht". It is suggested that MK dialect is part of "Estuary English" - a kind of watered down cockney - which some people believe will overtake SE.
See: PDF document Mobility, meritocracy and dialect levelling: the fading (and phasing) out of Received Pronunciation by Paul Kerswill