The idea of Britain being abandoned by the empire at some point is intresting, is there any time they Romans might have before OTL?
and if this was the case, would there be a chance for the creation of Roman-British sucessor state?
They abandoned Dacia, which was far richer, short before Diocletian.
They would have abandoned Britannia if it proved too expensive of if a major military menace showed off and they though it was the place not worth the effort keeping it.
I think the only reason why the Romans stayed in Britannia so long was that nobody threatened them there. AFAIK, the Anglo-Saxons made significant inroads only after the legions left.
If the Roman army leaves Britannia under or shortly after Diocletian, a Romanized successor state might consolidate, and will probably dominate all the British Isles in a century or so, though not by direct rule probably.
The Angles would face a far stiffer resistance, unless the Romano-britons collapse into internal strife like they did OTL. I guess that that less likely than OTl, but still a possibility.
In this case, more Germanic people will go for the Rhine. Saxons and Angles will push the franks in their rear or less likley merge with them.