Estuary English is to my ears a softer, more bland version of Cockney English. It intially spread from the slums of the East End of London after mass migration after the 2nd World war to the two counties bordering the Thames Estaury - Essex and Kent. This accent and variations now dominate those two counties.
Due to the mass media and further migration it has spread even further and has become the standard working class accent (among the young at least) for a large area of South-Eastern England.
I think it’s funny you should choose “hard” to describe the voiced sound in “the” and “soft” to describe the unvoiced sound in “thistle”. Would you call [d] hard and [t] soft? I would think of them the other way around.
Complicated accent anecdote: When I was a child I met an English child who had obviously heard that Irish people can’t pronounce the “th” sound, and would pronounce “three” as “tree”. (There is some truth to this, although the reality is a bit more complex). In the interest of testing this theory, he wanted to challenge me to say the word “three”. Except he pronounced it as “free”.
As Indistinguishible said, the “t” in such words as “metal” becomes an alveolar flap (voiced) in certain intervocalic (between vowels) circumstances. It is nota standard “d” sound. For me, the words “metal” and “medal” are homophonous. However, the “d” in “medal” is not the same as the “d” in “medallion.” They’re articulated similarly, but one is a flap, the other is a plosive. This is common in American English (from my experience, the majority of English speakers employ the intervocalic alveolar flap), as well as Australian and New Zealand English.
I don’t have any experience with this one. At least nothing immediately jumps to mind.
For the sake of precision, I’ll add that intervocalic alveolar flapping generally does not occur when the following vowel is stressed (so “buDAYduh” strikes me as quite odd, though perhaps it does indeed happen in Boston); however, in the contexts where it can occur, I believe it is just about categorical in North American English.
The two “th” sounds (voiced and unvoiced) are pretty rare in the world’s languages, so substitution is common, historically. In the various speechways that developed among enslaved Africans and their descendants, for example, the common substitution was [d]. In some Irish, from what I can tell, it’s [t].
(And yes, I realize I’m in danger of calling down a shitstorm on my head, I realize that it’s way more complicated, in both situations. Language is a extremely complex phenomenon. But my statements, while wild simplifications, are generally true.)
The most common substitution I’ve noted is “d” for the voiced “th” and “t” for the unvoiced “th.” So, “I went to da Nort’ Side to buy some tile for da batroom.”
I wasn’t complaining - except, perhaps, about the person who told someone they should ‘talk English in the first place’ - the real example of accent chauvinism.
Obviously there is no such thing as a ‘correct’ accent - it’s merely that the movement from one to another can be interesting.