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!