I reckon TTG should cast relative unknowns for their voices. It's great us fans being able to go "I want Hugh Laurie, and Johnnie Depp... and Brian Blessed... OOOH and Patrick Stewart!" etc. (Though, granted, we're not actually talking about such big names), but apart from a warm fuzzy feeling from hearing lines being read by guys we know/like, it serves very little purpose for a pretty big total hike in the budget. There are some extremely good voice-actors i've met in the UK who remain relatively un-tapped, and i'd be surprised if it isn't the case in the U.S.
I just want some Irish guy with dreadlocks and a purple suit, playing the piano in a basement bar (pub) voiced by Duke Special