unvoiced /th/ and /f/,Jennifer Rush Power Of Love

Solution 1:

You might draw a distinction between so-called standard English pronunciation, and variant pronunciations.

Any text book of English as a foreign language (no matter whether British or US or other) will have the same standard pronunciation. But the pronunciation ‘funder’ is common as a local variant, especially for people born in London and the South East. It is associated with what used to be called the ‘working’ or ‘lower’ classes. So it is frowned on in more formal discourse.

How it arises is obvious. It is the line of least resistance. ‘f’ requires less work for the tongue. We all do some of this. ‘Doesn’t’ for ‘does not’ is a short cut the middle and upper classes have accepted: ‘Funder’ for ‘thunder’ is not. In any case, ‘funder’ is a local phenomenon, rather than a national one. You will hear folk from Manchester and the North of England say “thoonder”. But this accent is not frowned on!