Further to what JustinB states, your original question relates to matching Nations in the game that were allies. Unfortunately, given the factional nature of Britain before the conquest (and after it, if truth be told), it would be nigh impossible to identify two nations that were consistently allied. In the Roman phase different peoples had different relationships to each other and to the Romans - some tribes welcomed and supported the Romans, whereas others fought against them. The game rationalises this by having three peoples occupying southern Britain and two in northern Britain - any of these could fight each other even after the arrival of the Romans.
The post Roman period saw the native inhabitants at each others' throats and worried about the Picts and the irish/Scots. The English Myth (supported by Welsh sources) is that the top dog amongst the British asked the Angles/saxons/Jutes to settle, following established Roman principles of federate forces being given land. The plan failed and the English peoples revolted and went on the land grab. This in time led to the "far more than seven kingdoms" period, often with English kings taking over native kingdoms. The "Angles" of the game represent three of the seven later kingdoms of the Heptarchy: Northumbria, Mercia and East Anglia, the Jutes were the kingdom of Kent and are thought to have settled the Isle of Wight and the coast of Hampshire, the Saxons represent the remaining Heptarchy kingdoms of Wessex, Essex and Sussex. All of these kingdoms could at any time fight another: Northumbria for example was formed after Bernicia and Deira fought each other and then went on to be the mortal enemy of Mercia (all part of the Angles in the game).
Similarly the Vikings were not exactly a unified force, although they did act together when it suited them, such as the Great Army. Contrary to what JustinB states, the Danes were not bought off at this time, that came much later. The survival of Wessex was a surprise reversal of fortunes.
In Scotland the situation is the same; the Picts and Scots were rivals, and the Picts were divided into at least two kingdoms, often more. The arrival of Vikings changed the dynamic so that in time the two peoples merged into one to fight the Vikings, although they came under the name of Scots (myths that the Scots wiped out the Picts are just that: myths).
So, in a sentence, no, not only is it impossible to find consistent allies, the game is balanced along lines that give players different spheres of influence and offer a range of possible enemies - it overlooks conflicts on a local scale in order to make the game playable.