Alright...perhaps that's an exaggeration. At least one actor, then.
I'm currently watching The History Boys, which features prominently Richard Griffiths and Frances de la Tour.
For god's sake, In Bruges had Brendan Gleeson, Ralph Fiennes, AND Clémence Poésy.
And you can't leave out Sweeney Todd with Helena Bonham Carter, Alan Rickman, and Timothy Spall. Perhaps not strictly a British film, but it does feature a mostly British cast (and I assume crew, as well).
The Harry Potter cast...they're everywhere!