Question Author
In view of the responses, I perhaps should reiterate the thrust of my OP - all the people I have listed have now, or in the past, obtained roles on both radio and television while, in my personal opinion, having none of the suitable credentials - a suitable voice and appearance and / or both.
That is why I wonder how they obtain their positions, when a pleasing appearance is usually necessary for long-term television success, and a pleasing voice is necessary for radio - these have neither!