A few times; talking about animation on The Big Breakfast, with my Womble collection on Collectors' Lot, in a Doctor Who monster costume in a Children In Need comedy sketch, and inside a Dalek for a BBC Promo (which you can view here)...
http://theherbs.homestead.com/BBCPromo.html
I was an extra (dressed as a doctor) in a Channel 4 drama, but I never saw it as they changed the title of the three-part drama.
Other appearences haven't been broadcast, I was in a Dead Ringers sketch that wasn't broadcast, and a made-for-video Hitch-hikers Guide to the Galaxy documentary had me dressed in the Marvin the Paranoid Android costume.
I'm also in a commercial that will be broadcast from January next year (though I'm only really a silhouette viewed from behind!).