Pronunciation, it's the key to being understood!

Call me picky, call me a nerd, call me an anally retentive pedant but I hate it when people don't pronounce words correctly. Regional, or even international, accents don't bother me, and I get that what is a bath up north becomes a baaaaath somewhere around Watford, but really some folks really can get my goat.

What I want somebody to tell me is when the diphthong th became optional in the way it's pronounced? I hear people use f, d or v all the time and nobody bats an eyelid (well, other than me) and nobody picks them up on it or corrects them. Well, I hear a few of you say, does it really matter, vere's nofing wrong wiv dat is dere? Well there is! (and not just 'cause I said so!)

Words can become other words, meaning can be lost and confusion can ensue. Imaging being offered a PlayStation Free in Currys only to be asked for three hundred quid when you get to the cash desk. All because somebody chose not to correctly form the word. What kind of car is a free one six i? What is wevver? I understand that you're saying it's raining, but I don't remember this wevver stuff ever cropping up.