Whoa! Merry Olde England had a king (several of them at the same time, as Arthur was a king only in one area: http://en.wikipedia.org/wiki/King_Arthur
Anyhew, Arthur was king; under him were the nobles; under the nobles and fighting for them were the knights. The supreme power then were the clergy.
So, even if it is true that you are descended from Tristan, you would not be descended from nobility. The poor folks had 2 ways of improving their lives: join the clergy, or become a knight (that is a mercenary in more modern terms. Of course, the third way was to be a bandit or pirate....
The legend of King Arthur and his Knights of the Round Table has never been proved, although there is much evidence to lend credence to the story. It is like the story of Robin Hood (I am descended from a Robin who lived in Sherwood Forest...it is always fun to think, Wow! Maybe he was an ancestor! But, again, the story of Robin Hood is just that: a
story. Probably also backed by facts, a real-life person, just not like the stories handed down to us.
If being descended from Nobility is your bag, don't sweat it: you are. No matter where in the world you live, where in the world your ancestors came from, some where along the line you had noble ancestors.
E.g., I am descended from Charlemagne, through 3 different lines that I know of. That means I am also descended from kings & queens in every country in Europe, as well as Roman caesars.
Now-a-days, in the U.S., the poor have the most children. Why not? They do not need to work; the Government supports them. Same with all the Illegals (now more than 60,000,000 living in the U.S. --- www.census.gov), but before, if a poor couple had children, most would die before reaching adulthood. Kings & queens & nobles had dozens, even hundreds, or thousands (just google King Louis, esp. Louis XIV). They had plenty of food, the best housing, and what passed as medical care; most of their children survived.
If you have truly searched your genealogy, you already know this. Until the late 1800s, the most common cause of death for women was childbirth, for men, "accidents" (being kicked by the mule, horse, bitten by snakes, chopping a tree down and its falling on him, etc.)
Read books, such as this one: “Mapping Human History: Discovering the Past Through Our Genes” by Steve Olson, who claims that everyone in the world is descended from Nefertiti, Confucius, and Julius Caesar. Other genealogists add Mohammed the Prophet.