Usually we think of patriotism as a positive thing: it's healthy to be proud of your people and nation. But patriotism in this novel often comes across as negative and intolerant. Cedric and Ulrica loathe all Normans. (To be fair, we can see why: they have seen terrible violence in the aftermath of the Norman Conquest.) This strong sense of Saxon pride and anti-Norman hatred leads Cedric to disinherit his son and Ulrica to lose her mind with bitterness, which are obviously bad things, right?
Keep in mind that Scott is writing at a time of strong resentment between France and England. Ivanhoe was published in 1819, just a few years after the Napoleonic Wars, in which England finally kicked France's butt at the Battle of Waterloo. Against this historical backdrop, the struggle in Ivanhoe between the snobby, domineering Normans (French) and the plucky, honest English guys (the Saxons and the outlaws of Sherwood) takes on a whole new pro-English meaning.
But Ivanhoe is not a simple tale of England-versus-France. After all, that great king of England, King Richard I, is himself a Norman, and the book clearly disapproves of Cedric's knee-jerk anti-Norman feelings. Indeed, there are some real meditations on what it even means to be English. It's not like the Saxons were the first people in England. What about the Celts – the Irish and the Picts and the Welsh? What happened to them once the Saxons came sailing over to the British Isles?
At the end of Ivanhoe, the narrator admits that the differences that seem so sharp at the beginning of the novel between Saxon and Norman totally break down by the time we get to the 14th century. Historically, this whole Saxon-versus-Norman problem disappears, and everyone just starts speaking the same mixed language and basically getting along. Which seems good to us. What's the point of either Norman or Saxon patriotism if the only thing it does is encourage people to fight each other?
Scott uses the patriotic struggles between the medieval Saxons and the Normans in Ivanhoe to point out the diversity of the supposedly unified "English" identity.
Ivanhoe presents the fierce patriotism of characters like Cedric as a throwback to earlier models of identity. The younger generation of characters – including Ivanhoe, Rowena, Athelstane, and King Richard I – are all more capable of compromise.