The Last Kingdom is fairly accurate historically, other than Uhtrid is not a real person. Alfred the Great actually did hide in a swamp for months after getting ambushed by a Viking raiding party at his capital at the time. That said, the historical record of the period is fuzzy because only about 1% of the population in England could actually read and write, so there are only a handful of records of what happened.
I always had to teach a class a year in ancient world history and western civ. I'm still surprised how little was mentioned about the Vikings in the book used.