I do not think you will be happy taking only head shots on fox or coyotes with the 17. The result you stated in your first post seems to be typical in my experience. I've had that small bullet break apart one the front teeth and not reach the back of the throat also - only broke one tooth then turned to dust. It did knock the coyote cold so ten minutes later a second shoot was needed.
Perhaps a heavier bullet would help - maybe the 30gr golds - I've never tried them on facing head shoots....
Keep us updated on your progress. BTW - From the photos I've seen - those UK fox look like they are about the size of SW US coyotes - huge compared to the red fox I remember from the east coast of the US.