She did a decent one in "Possession" (good flick, also has Jeremy Northam, Jennifer Ehle, and Aaron Eckhart.) Good enough that while it was a teensy bit stilted, it didn't stick out (and that worked for the character. I don't think that was intentional as she sounds the same in other movies with an English accent, but it worked.) And I've found the reverse--a lot of American actors can learn to do at least a functional RP English accent, there's often something not quite right about an English actor trying to sound American-something about the vowel switching seems to be harder. Hugh Laurie was notable because he was VERY good (I can hear a LITTLE bit that the accent isn't quite...something...but that's because I've seen him in A LOT of other things and know to listen for it.) And Scottish actors, well...some have trouble sounding ENGLISH, let alone American. Watch "Rome" and listen to Kevin McKidd every time Vorenus has to be angry-accent runs North to the border. Apparently it was an issue for David Tennant on "Doctor Who", too--he made the mistake of complaining to the writers about "oo" sounds, which are apparently REALLY hard to fix for someone with a Scottish accent. Next script, they gave him the line "Judoon platoon upon the moon." I wouldn't have blamed him from whacking whoever wrote that over the head with the script.