I've only recently read them all, but for the last two. Sean would be an awesome mature King Alfred or the priest- mentor, Father Beocca, but the first two books at least would require a much younger actor to be Alfred. The lead role, Uthred, needs to be someone not only younger, but not too-young, but in a good physical condition to step into the shoes of the young, blond, blue-eyed warrior. I can picture Jai Courtenay taking up the role. He made a stir in a gladiator loincloth in Spartacus: Blood and Sand and is currently finishing up the Kyle Reese role in the Terminator reboot. There are so many roles to be cast that, while meager in comparison to GoT, would still be quite substantial, more like Vikings than Downton. I can imagine a lot of actors who have had their characters killed off on GoT and Vikings finding roles in this, especially given as there are more than a few characters that make appearances in Vikings, which starts out shortly after Ragnar Lothbrok's time and his sons have prominent roles in the middle 3 books of the series. It will also be a draw for actors coming off of the Spartacus series. The books have a huge following, nothing like GoT, but a solid audience-in-waiting and given the nature and popularity of the Viking genre at present, this has HUGE potential. Under the careful hand of the BBC it promises to be a huge hit on the scale of Sharpe as far as UK audiences go.
It should be easy for Sean to work out filming the Alfred scenes in between doing Legends for as long as Legends runs. There are currently 7 books out and the concluding book will be out this October, so it should have a nice run.
For those who might like to read them, be advised, get the whole series before you start. Once you start you'll burn a LOT of midnight oil as they are incredibly addictive, perhaps a bit more so than the Sharpe books. They cover a lot longer time period in a shorter number of books but with just as much intensity. And don't be surprised at the similarities between Uthred and Sharpe.