That's a good example. James Corden, Smithy, was in the stage version of The History Boys & Gavin (forgotten his real name) had already done seasons of the Catherine Tate Show. Not to mention La Steadman, and Julia Davis in a tiny role for someone who had her own show (& a part in Four Lions).
In this regard US Comedy and tv drama is miles ahead; huge casts of "jobbing" actors ( a lot of Brits as well- Hugh Laurie, Olivia Williams, West & Elba in The Wire), so you get Bryan Cranston say popping up all over the place in films; adding that touch of class to whatever he does even if its a bit part.