Did the various Germanic peoples all become one nation? No, so why would these various Celtic and Romano-British people unite?
Also, I think you spelled "Celtebria" wrong, because it gets only ~3 results on Google, so I have no idea what you mean.
The best way for this to happen would be that the Romans conquer and Romanize Caledonia (Scotland) and Hibernia (Ireland) in the 1st-2nd centuries AD. This could lead to a totally Romanized Britannia which may end up united enough to fight off any Germanic invasions that will probably not come (no need to invite Angles and Saxons as mercenaries, although Frankish and Frisian pirates will still be a problem).
Maybe in an alt-Crisis of the Third Century there is a Britannic Empire which manages to hold Gaul and Hispania too. Later Frankish invasions defeat Britannia and the empire loses Gaul and Hispania. They manage to keep Armorica out of Germanic hands, and it remains integrated with the Romanized Britain.
But you seem to be asking for an England that exists AIOTL, which makes it almost ASB for the other Celtic peoples to unite then.