From my standpoint you are the confused. I'm looking at it in the 3rd person. What the alliances were in this ancient time is unclear. Fact is that the Britons were in an anti-Angles coalition and the Angles came with their whole volk to defeat them. If the Britons had contact to the former Roman Empire is unclear but possible. They lived under Roman rule for centuries and were romanized. This defeat was something like de-romanizing to them after 500 years of Roman rule. This can clearly be seen when you read and understand the Roman Britain-Anglo-Saxon battles in Germanic Wars.