I think it stems from the launch of Eastenders in the mid 80s.
Until then, nobody north of Watford ever thought of substituting a ‘th’ sound for an ‘f’.
No matter what rough estate you were from nobody said ‘fanks’ and ‘fink’….. this started to creep in early 90s and has now made nationwide coverage of what was originally limited to cockney and the south east.
It is now so normal that foreigners learning English speak it, and BBC radio are happy to employ those who speak it, the loss of the ‘ing’ in favour of in’ has followed suit until there is a full house of bastardised language everywhere.
Local dialect or slang is fine, but this is neither, it’s simply imitating the tv.