Ummm... I'm not the world's greatest expert on A/S, Sen, but I'm not aware of any relation between 'th' and 'f'; between f, v, & w, yes, but not th. And forest, of course, comes to us, via old French, from Latin, so isn't an A/S word (though I realise you're commenting on the pronunciation rather than the etymology). Ford is English & directly related to Dutch voorde (f-v-w, but not th).

Your 'th' may be dialect (and I don't dispute that you've heard the substitution), but I don't fink (sic) it's a holdover from A/S. Incidentally, the accent & pronunciation that is said to be closest to A/S is, I believe, Leicestershire...