Re the movie...okay, she'll do for Cleopatra; however, I can't think of ANY Brit nor American actor whose acting "chops" can pull off Julius, Mark, or Octavian/Augustus. Perhaps it's because I've seen truly FANTASTIC actors portray those men in the past. so am a bit "jaded". OTOH...there is NOT a single American actor who could fill any of those roles; not a one!
I'd say that Ian McShane could easily do a great Julius, but he's too old now; sadly.
Not surprising since in the last 2500 years there have only been a handful of men that had the stature of a Julius Caesar. Enjoyed the conservation.