pat9
It is Lazy speech. You have to use your tongue to pronounce T and other letters. Now we are losing L - hospiaw, medaw etc How can children learn to spell correctly when words are not pronounced properly? BBC announcers and presenters should use RP so that standardised pronunciation is available to be heard.
Dialect is a different matter and can be delightful. It has been around for a long time and is part of the variety of the country "Estuary english" is recent. I never heard it when I was young.
I'm 59 and have always spoken the same, having been bought up in the Thames estuary area.
I haven't invented it all by myself; all of my family and everyone I've met here speak the same.
Originally people were from London, so parts of the speech is cockney.
When I heard a someone with an accent I couldn't place, and asked where the woman came from, she told me she had a proper essex accent.