First a general question: What were "legions" like by AD 400? My impression is that the the classical legionaries, Marius' Mules and the regulars of the early Empire, had long vanished by this time. If the "legions" remain in Britain, are they really very different from the auxiliaries who did remain?
To the point: My recollection is that the last would-be usurper took the legions with him to Gaul, so they weren't so much "recalled" as simply never sent back. When the Britons asked for them back, Rome (Stilicho?) said, "you're on your own."
Suppose that, instead of marching away with them, someone proclaims himself Emperor, but chooses to stick tight in Britain. This had been tried before, by Carausius c. 280, but by c. 400, what is Stilicho or anyone else going to do about it? If in OTL there weren't legions available to re-garrison Britain when it asked for them, there certainly aren't legions to spare to re-conquer it from a breakaway usurper.
Perhaps our guy has a British mother or wife, which is why he doesn't cross over into Gaul, even though it's much richer. Or he reads the handwriting on the wall - if he crosses over, he'll just get caught up in the general turmoil.
Does this give Roman Britain the resilience to resist or absorb the Saxons, and avoid fragmentation into quarelling post-Roman kingdoms? Does Britain end up speaking neither English nor Welsh, but a Romance language akin to French?
Or is Roman Britain too far gone to be saved? Romanization had always been pretty thin in most areas - it's indicative that Latin died out in Britain, whereas it survived even in northern Gaul.
-- Rick