θ is a weak sound, and weak sounds tend to slide down a gradient into silence or into stronger sounds. θ goes to f in many British dialects. Lately I’ve been noticing θ going toward t in some proper London types. This might be an old tendency but I haven’t noticed it before.
It’s constantly audible in the deadpan financial commentator Patrick Boyle. Now I’ve heard the same shift in the narrator of a piece about the Royals at DailyMail.
Polistra's Mill 