A Mythical "Poldark", Filmed in the 1950's!!
I believe that Winston Graham's saga didn't have the popular, nor critical acclaim it deserved, when the first volumes were published, in the '40's and early '50's.
Let's now then assume, that the Poldark Saga, had caught on, like prairie fire, as it was published, and people were clamoring for it then, to be made into a Silver Screen Movie/Silver Screen Movie Series/early TV dramatization.
You know, there have been highly-skilled British Actors and Actresses, who've created American characters, immaculately. Our own Robin, was a fine American, in 1980's The Europeans. My favorite American character of Jane Seymour's, was Natalie Jastrow-Henry, in War and Remembrance. She even used a bit of the Bronx twang, for Natalie.
And, there's even been a few American talents, who've created British characters. We have no farther to go, than our own Jill Townsend, who was born in, and grew up in Santa Monica, Los Angeles County, California, USA. She made frequent appearances in Westerns in the '60's. And, no less, she was a "Westie Babe", in, Robert Conrad's The Wild, Wild West, just a few short years, before she became our Elizabeth. So absolutely convincing as an Englishwoman, that many years later, I was astonished to learn that she was American!
EDIT, AS OF 13/08/15: And another fine American, who effectively played against nationality, was Gregory Peck, as the dreadful Dr. Josef Mengele, in The Boys From Brazil.
All this said, there was an American talent, who, if he were successful in achieving English/Cornish expression, would have made a very fine Ross Poldark, back in the '50's, and that would be, William Holden. So, grittily "macho", yet could pass with some ease, amongst the/his Gentry class.
EDIT, AS OF 13/08/15: Yet for you British Talent purists, I would recommend Richard Todd. Also, if he were just 20 years younger at this time frame, Errol Flynn--an Aussie--minus the moustache.
Demelza? I have in mind two ladies, who'd actually be more in line with Mr Graham's physical description of her, [EDIT, AS OF 13/08/15:] than was Angharad, nor Elinor. The lates Dana Wynter and Audrey Hepburn.
Elizabeth? Honor Blackman (Goldfinger), or the late Nyree Dawn Porter (The Forsyte Saga).
Francis? Roger Moore, or Hurd Hatfield. EDIT, AS OF 13/08/15: Mr. Hatfield may have been American, but by the 1950's, he was successfully "ex-patting" himself to the UK; in the main, he was always more comfortable with the British way of doing things.
Verity? Angela Baddeley (Upstairs, Downstairs) or Margaretta Scott (All Creatures Great and Small, "Mrs. Pumphrey").
Dwight? A very, very young Sean Connery.
Captain MacNeil? Patrick McGoohan (Danger Man/Secret Agent Man), Gordon Jackson (Upstairs, Downstairs), or James Doohan (Star Trek). EDIT, AS OF 13/08/15: Malcolm ought always, to be played by an ethnic Scot/Celt.
George? The lates Sir Christopher Lee, Sir Alec Guinness, or, Eric Porter (The Forsyte Saga), or, playing slightly against type, David Langton (Upstairs, Downstairs).
Monk Adderley? Either Kenneth More, Bernard Lee, or Patrick MacNee (The Avengers).
Caroline? Glynis Johns--she always impressed me as an older version of Judy, her crackly voice, general appearance, and pert, cheeky ways.