There is a large number of British actors who are very adept at American accents. In addition to Hugh Laurie, there is Jamie Bamber who played Apollo on Battlestar Galactica. Another is Dominic West who played McNulty on The Wire. I had no clue West was English until I heard his commentary on The Wire DVDs. (I knew Bamber was English from his turn in the Horatio Hornblower series, but otherwise I’d never have guessed.) There are others, but those are the ones at the top of my head.
On the other hand, I can’t think of any American actors who do a truly convincing British accent. (I was going to use Keanu Reeves in Dracula as an example, but he is so utterly bad in everything, he can’t be taken as typical.) Some are passable, but you can still tell (I’m thinking of James Marsters who played Spike in Buffy the Vampire Slayer; he was pretty good, but slipped up with some regularity.) I think it must be something in the way actors are trained in the two countries.
Regardless of the medium, it is absolutely imperative to employ a British accent when depicting a Nazi or Roman. ;-)
It is acceptable to portray a Nazi using a bad German accent. And Tony Curtis made a great Roman in Spartacus, but he is sui generis and “was taught de classics.”