What if the Angles, Saxons, and Jutes failed to conquer a large foothold in Britain, so they migrated the other direction, settling the lands east of the Elbe? As far as I know this area was partially vacated by Germanic tribes migrating into the Roman Empire, and quickly settled by West Slavs.
Let's say that the Angles, Saxons, and Jutes topple the Veneti and take over or establish hill forts along the Elbe, Oder, and Vistula. Anglo-Saxon statelets form similar to those in Sub-Roman Britain. Also, there is interaction with West Slavic confederations (like the Obotrites) that form more quickly in response to the new threat.
How would proto-"England" develop differently being so close to Old Saxony itself, with reduced influence from Rome and Roman infrastructure, and also with a Slavic instead of Celtic substrate to Old English? How is Poland going to develop differently, as well, if it still exists?