It's not possible to make a realistic currency system for Brytenwalda or even native Warband. The barter system was a major part of the economy, with the weight and composition of coins being the biggest determining factor in their value. So many different types of coin were minted by different rulers that even the most basic exchanges became incredibly complex. This problem wasn't ironed out until much later, in the 1600s-1700s, and even then the barter system was still pretty important. "Tithes" are originally, for example, bags of dried grain given to the church for the churchmen to eat. Taxes were levvied on peasants in much the same way.
Edit: What I should say is that we have to abstract the currency system for any game not based in prettymuch modern times.
Great thread so far. Good historical fiction novels that cover this span of time include Bernard Cornwell's "The Winter King" series. Might help people add some colour to their characters when they think about Brytenwalda.