There was a pretty big rivalry between the Angles and the Saxons. What isn't shown in the game is that the Saxon culture consisted of many different "nations": Wessex, Essex, and the like. I believe the Angles were also set up in different nations/tribes. However, just like in the game, it's useful for the Saxons and Angles to not annihilate each other, lest the Danes completely take over the island. Historically, the Angles were devastated and the Saxons had to mount a defense. What can't be modeled in the game is that the Danes usually were bribed to not attack until Alfred the Great resisted them and pushed them back, reconquering much of the lost Saxon territory. What also can't be shown is that eventually the Danes settled down and became part of this confederation of cultures. Due to a weakening Saxon line of Kings, the Danes actually held much influeIt is actually fairly ironic that if William is killed the game ends, as it is hard to imagine Harold or Harald putting so many to the sword and working so hard to hold territory. In the process of this he unified much of the island in a way that hadn't been achieved before.
Anyway, to answer your question, the alliances are mostly represented by the tribes, such as "Belgae", or "Picts", but especially the Saxons and Angles. For the most part, every token on the board represents a local tribe with its own organization and its own leader. The symbols on the tokens show that all of these tribes share a common cultural link, but there was certainly much infighting and attempts to seize power. For the most part I would say that there were very few permanent alliances. As for mortal enemies, Boudicca's daughters were raped at the orders of the local Roman governor. This was, according to the history books I have read, a rogue action, but all of Roman Britain suffered for it. (And understandably so, it makes me very sympathetic to Boudicca). The Picts frequently raided the Romans, which is why they built Hadrian's Wall. The Picts made a charge into Roman territory at least once on a major scale. Eventually, they were assimilated fully by the Scots, and Pictish culture disappeared. The Welsh largely sat content in Wales, occasionally making trouble for Rome or the Saxons. The Welsh had the most to lose from Williams' invasion. I suppose if the game went longer, the Welsh would seriously consider helping the Saxons!
The Danes, of course, raided any monestary they could find, so almost everyone hated them. The Norwegians, Normans, and Saxons were all three parts of a succession war, and their introduction at the end of the game represents a big step in Britain's transformation from tribalism to nationalism (for the elite, at any rate) and their tokens should not be considered as individual tribes, but as regular armies.
Well, now I have to read my English history books. I had a class that briefly covered this time period, so I may not have everything 100% accurate. Unfortunately, I don't have them handy, so you'll have to trust my memory. Also, I'd look up the other groups for you (though mostly this particular book only deals with England). I hope that gave you a bit of what you were looking for. If you're interested, I can tell you the name of the book, I'm certain that it's available online or some library or a bookstore.