Start in Scandinavia as Duke Björn Ironside of Uppland, build up your empire and swarm Britannia once you're strong enough. Starting in England as Ivar or Halfdan will get you stuck between strong kingdoms, you'll have issues with religion and culture. Scandinavia is a lot more homogenous in that regard. You only have to start as one of the characters, you may of course die and continue with your heir. I finished around 950~.