My 'lack of succinctness' worried me when I posted! I'll try again: Mike Carson's voice is used by advertisers, but not on ITV because people who work as ITV announcers can't also be heard in ads on the channel.
Presumably because Mike Carson works as a continuity announcer (and/or voiceover artist for trailers, etc) for ITV (and many other channels), with the 'conflict of interest' rules preventing his voice from also being used on adverts on ITV (even though he does corporate advertising work elsewhere).
http://www.voiceoverman.co.uk/page3.html
My 'lack of succinctness' worried me when I posted!
I'll try again:
Mike Carson's voice is used by advertisers, but not on ITV because people who work as ITV announcers can't also be heard in ads on the channel.