What are they saying instead of th, when did this start, and is it particular parts of Britain or particular social groups?
(Using f/v instead of th has long been a feature of some London dialects, and using t/d instead of th has long been a feature of some Irish dialects, and young children often use f/v instead of th, I think. Jocular references to "the youf" have been around in print for a while.)
EDIT: Of course there's also Jamaican pronunciation with t/d instead of th!
As an aside, the dialect I used in my youth used "f" a lot - fit, fa, fan and far for what, who, when and where. Leading to "Fit fit fits fit fit" making sense...
(Using f/v instead of th has long been a feature of some London dialects, and using t/d instead of th has long been a feature of some Irish dialects, and young children often use f/v instead of th, I think. Jocular references to "the youf" have been around in print for a while.)
EDIT: Of course there's also Jamaican pronunciation with t/d instead of th!