I don't doubt your level of education at all but, out of curiosity, can you actually pronounce the "th" sound "correctly" (I can't think of a better way of putting it) when you try?
Yes, I can. I tend to use the /v/ and /f/ for 'th
' only when in the company of others who do the same. I grew up in a lower-class London (Souf
London, in fact) environment. I rarely speak in this way nowadays as most of the people I mix with come from different regions and social backgrounds, but occasionally I meet somebody with whom it feels right to speak to in this way.
I believe that the way a person speaks is very much a part of their identity, which, although far from rigid, and tremendously complex, is essentially rooted in the social group to which one feels one belongs. (That's what I meant by "my most natural speech".)
Of course, when I'm in the classroom I speak with the most utterly flawless RP. ;-)