Dialects, Accents, and Change

Here's a question.

Dialects clearly change.

Presumably cinema, radio and TV should to some degree stabilise dialects and accents.

Yet with 80 years of taped vocal sources is it clear that in the 20th century, in the US and in England (and I assume other areas), accents, even when constrained by class and area, have changed very very quickly.

I suppose Elizabeth II is one of the most recorded people ever, and she has remained her entire life within very limited class and geographical limits. But as far as one can tell, even though she is very much much retrograde, her accent, along with most of the British aristocracy (cf. The Duchess of Devonshire, one of the Mitford women, and almost as recorded as the Queen), has changed in what seem basic ways in the past 50 years.

East London White Working Class accents (aka Cockney) are very different now from those recorded in the 1940s. [And reading Charles Dicken's renditions of Cockney, in say Bleak House or Martin Chuzzlewitt this change was just as fast in the previous 60 years. When exactly did East Londoners stop saying "wittels" for "food" for example?]

There are similar changes in recorded Mancunian and, to take another case, Dublin, accents.

So here's the question. Do accents change faster now than in the past, or at the same rate? And are accent change rates a measurable quantity?

Up until recently I have always understood that there was some major post-Shakespeare shift in, say, East Midlands, English pronunciation, but that that was a relatively rare event.

What work is there on this?