The show was shot on Super 16 AIUI and didn't air on BBC HD in the UK. (The BBC HD channel won't air, or fund, shows shot primarily on Super 16)
I don't know if the show was posted in 1080/25p for 576/50i downconversion, or posted in 576/50i. If it was posted in 576/50i then either NBC have an upconversion OR there has been a reconform with a re-telecine of the Super 16 to HD. I'd be surprised at the latter.
My money is on a 576/50i upconvert.
I enjoyed the first series in the UK - good cast and a great location in France, so less CGI and more reality. (The chateau they used was actually quite new in places - so the stone is clean and looks new, as it would have at the time)
As for it being that close to the original Arthurian legends... Naah - it's a TV show with a few common threads. Enjoy it for what it is. A bit like Robin Hood!