Is it a general rule (I've never noticed it--) or a sometimes rule?

I don't know if you have ever seen, or have access to, a BBC soap called EastEnders (it fil ne bet ne werse than any others), but, exaggerated though it may be, the accents pretty accurately reflect the East End/near Essex accent of which (along with RP) Estuary English is the bastard child. The point is that the 'f' and 'v' thing is virtually a rule:

bruv/bruvver (brother)
fick (thick)

I have also noticed, over the past few years, the tendency of those with sarf Lunnon accents to slur their esses ('s's), so 'stupid' comes out 'shtupid' and so on. This is a tendency particularly prevalent amongst teenage girls (in my experience) and women in their early twenties. Has anybody else noticed it? Is there an explanation?