Unfortunately, it appears that "The Last Kingdom" is much more of a historical fiction series than an authentic retelling of England's history. During an interview with Den of Geek, the historical advisor on "The Last Kingdom" (Professor Ryan Lavelle) compared the show's portrayal of history to a compromise between fact and fiction. "The evidence for our period is limited, so we always have to play with history to an extent," Lavelle explained.
According to Lavelle, a few of the more glaring historical inaccuracies within the series include the show's extremely condensed timeline of historical events (which makes some characters far younger or older than they would have been in real life) and the invention of a plague during Season 4 — which has a very weak historical source, and which absolutely did not happen at the time which it was depicted. Furthermore, many of the character personalities within the series are an invention of the writers: such as the bloodthirsty Aethelred of Mercia (Toby Regbo), who is an utterly despicable villain in the series, but who was likely an ally to King Alfred according to real history (via Den of Geek).
Perhaps the most egregious historical inaccuracy in the series is the character of Uhtred himself, who is a fictional character created for Bernard Cornwell's "The Saxon Stories" and who is loosely based on the historical figure Uhtred the Bold; who lived during an entirely different century than Uhtred from the show. Ultimately, it's clear that "The Last Kingdom" is a blend of both fact and fiction — which is perhaps a good thing in the end, as this added drama helps the show to remain captivating and exciting.
