It depends. I would think not.
We're talking of a region that was going to be sooner or later abandoned by Rome. Once imperial control history, the local britto-roman would likely react as OTL, not willing to let foreigner takeover their lands : not only by the original settling but by their likely expansion due to the imperial withdraw.
Not christianized, not being at the direct contact of Rome, more close to northern Germanic features than, say, Franks...
That's certainly putting them in a position where romanisation isn't a viable option. It would be too late for that.
You'll need a PoD where Saxons are at least christianized in the III century and being more migrant people than pirates (OTL they settled on North Sea and Channel shores, being quickly merged with romanized germans in Gaul).
Finally, you need an incitative to make them foederati : the main threat against Britain were Saxons, so at the very last, you need some Saxons ready to fight them and not being part of the raids.
Being a loose confederacy without clear leadership, I doubt you'll find easily a clear leader figure able to gather forces to fight against other Saxons.