Athelstane, it is true, was vain enough, and loved to have his ears tickled with tales of his high descent, and of his right by inheritance to homage and sovereignty. But his petty vanity was sufficiently gratified by receiving this homage at the hands of his immediate attendants, and of the Saxons who approached him. If he had the courage to encounter danger, he at least hated the trouble of going to seek it; and while he agreed in the general principles laid down by Cedric concerning the claim of the Saxons to independence, and was still more easily convinced of his own title to reign over them when that independence should be attained, yet when the means of asserting these rights came to be discussed, he was still "Athelstane the Unready," slow, irresolute, procrastinating, and unenterprising. (18.37)
To sum up: Athelstane may be of royal Saxon blood, but he's lazy and unlikely to ever fulfill Cedric's dream of getting a Saxon king back on the throne of England.
In Scott's first novel, Waverley (1814), he depicts a real historical character named Charles Edward Stuart, popularly known as Bonnie Prince Charlie. Like fictional Athelstane, real-life Bonnie Prince Charlie was the descendant of a family with a claim to the English (and the Scottish) throne, but his family was put out of power by the English Parliament in 1688. In 1745 Bonnie Prince Charlie, with help from some Scottish gentry and troops from France, tried to attack England and regain the throne. He was defeated and fled in fear back to France. This uprising was called the Jacobite Rebellion.
We bring up Bonnie Prince Charlie because, in a lot of ways, his character in Waverley seems to be the model for Athelstane. Bonnie Prince Charlie has nicer manners and more energy than Athelstane, but he is also self-centered and vain. And like Athelstane, Bonnie Prince Charlie is never really a serious contender for the English throne.
There are actually a lot of echoes between Waverley and Ivanhoe. Both focus on romantic but ultimately doomed resistance movements. And both depict the struggles of living under colonization: in Waverley, it's the Scottish Highlanders under the English, while in Ivanhoe, it's the Saxons under the Normans. The two novels may have very different settings (see "In a Nutshell" for more on these differences), but clearly Scott understood what themes the British public wanted to see in their novels.