I have a relatively simple question; what are the likeliest PODs to stop Anglo-Saxon dominance over the British Isles? This would not have to remove Anglo-Saxons completely, just leave 'England' controlled by Brythonic peoples, with a Roman-Brythonic culture. Since we're dealing with the very stereotype of a dark age here, there may be a lot of guesswork involved, but i'm perfectly okay with speculation going on here.
Also disclaimer; This is NOT done out of a nationalistic wish to have a 'pure British isles' or any of that nonsense.