Julian McMahon (who is from Australia) and Joely Richardson (from England) as Americans. In general, Richardson's American accent is much more accurate than McMahon's (except when she says the word "family"). Also, Rhona Mitra who plays Kit McGraw is revealed to be American, although Mitra is English.
Averted and then played straight with Kit McGraw. Rhona Mitra used her real British accent in playing the character, but in her last appearance, the character was revealed to be faking it, and used an American accent.
Fake Brit: Inverted with Jude Watson and Kit McGraw. They both play as an American character who are faking their British accent for one reason or another, but both Phillip Rhys and Rhona Mitra were born in London, so the accent they're "faking" in the show is actually their authentic accent in real life.