Since British actors tend to work a larger range of mediums (although that is changing), they turn up all over the place. Stephen Tompkinson from Ballykissangel showed up in a reader's theatre version of The Sittaford Mystery. Jonathan Firth (no relation to Colin) from the Brother Cadfael mysteries shows up constantly as bellhops and clerks and once as Edward VI. And the odd but definitely gifted Samuel West seems fairly ubiquitous.
Extras are just way, way more fun than stars.