What is happening when a sound in RP English usage is non-strident [ð] is replaced by a strident sound [v]? For instance, the word 'Father'.
[v] and [ð] are acoustically very close together. Jongman et al. 1998 note that the frequency of the spectral peak for labiodental and interdental fricatives is not distinct (but at other fricative places, for z and ʒ, spectral peak frequency is different). They are also the same in terms of duration (they are shorter than the sibilants). Noise amplitude is what makes [v] and [ð] distinguishable -- [v] is louder. A frequent form of sound change is where two sounds are acoustically very similar, meaning that they can be hard to tell apart in sub-optimal conditions (i.e. normal life). Once these changes start, they can spread like wildfire.