I always assumed that the use of would of instead of would have was a comparatively recent error, perhaps due to mishearing would've. In general I like colloquial language as an aid to understanding without pomposity, but I'll admit that errors such as would of do grate. To my surprise I recently encountered this in my holiday reading - a compilation of Poirot short stories.
Ever so fond of Mrs Oldfield Nurse was, and ever so distressed when she died, and Beatrice always said as how Nurse Harrison knew something about it because she turned right round against the doctor afterwards and she wouldn’t of done that unless there was something wrong, would she?’
Agatha Christie - The Lernean Hydra (1939)
So not recent after all. This is not Poirot speaking of course. Presumably it's Aggie's way of indicating an education not out of the top drawer.
Incidentally, I think David Suchet's moustache in the TV series is not quite as large or forbidding as it should be.