Even though the Saxons were stopped and even pushed back after Badon Hill and there wasn't a Bretwalda for quite awhile, the Britons didn't seem like they were in a position to drive the Saxons back into the sea.
The Wiki seems to think that after Badon Hill, the Britons controlled a salient that kept the Saxons geographically divided. Assuming "Arthur" in TTL doesn't have a "Mordred," we might see a united sub-Roman Britain and two separate Saxon states.
Of course, even if the Britons stay united, between growing numbers of Saxon immigrants and the Yellow Plague, they might end up being pushed back anyway, just slower and less thoroughly.
Bigger Wales?