From my standpoint you are the confused Im looking at it in the rd person What the alliances were in this ancient time is unclear Fact is that the Britons were in an antiAngles 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 deromanizing to them after  years of Roman rule This can clearly be seen when you read and understand the Roman BritainAngloSaxon battles in Germanic Wars