I don't know why they don't just import the original British version of the show.
Actually, this is one of very few cases where I think there's a legitimate case for making an American remake of a British show (unlike, say, the attempt at an American 'Red Dwarf'). This show is essentially about the culture shock between time periods, and since American and Britain had/have different cultures (particularly in the Los Angeles region, which is where the new show will be set), the American version--if done well--won't just be a retread but should have all new vistas to explore usign the same premise. What I don't get is why the American show is sticking with the early seventies timeframe as the British counterpart - it seems to me that placing it in the mid-sixties, contemporaneous with the civil rights and counterculture movements, would have a lot more dramatic potential.
As for the actors - they actually tried to get the British leads to come over and do the American version of the show, but they were busy with other projects. So the original idea called for an even more international cast.
Fictitiously yours, Trent Roman