Other than the colony itself I'm not sure how accurate the show would be in any form. Jamestown was set up in 1607 and the show takes place in 1617.
Which is kinda strange because like I said there is a lot of early non-fictional material that would lend well to a gritty dramatization, such of the story of a young Pocahontas saving Captain John Smith from getting his head bashed in by Powhatan natives.
I think the creators of Downtown Abbey are just cashing in on their success and repackaging it. Which is sad because it's a remarkable story that not many people know about and if HBO/AMC were to pick it up it could be an incredible piece of television.