After the Saxon invasions of Britain had begun, was there any way the Britons could have either pushed the invaders back into the sea, or held a united half of the island against them? Most I have read about this period makes the Saxon domination seem inevitable, but I have been wondering if there was a chance for the British. Is there Any way Dunnomia/Gwent, etc could have prevailed in this struggle?
I believe you mean
DUMNONIA, its the ancient root of Devon btw.
And as for your question - Internal divisions were rife at the time, so many separate kingdoms. Its the same reason the Romans managed to conquer the British Isles. Some fought, some surrendered, some became allies etc.
Also bear in mind it wasn't just the Saxons. You had the Angles, and Jutes, a number of different peoples coming to Britain at this time.
And I would argue several places (Cornwall, Wales etc.) did manage to retain their own individual culture in the face of the Saxons. I don't believe in the idea of 'Celtic Genetic Purity', every nation in the UK today is a huge mix of peoples from over history. I'm sure the 'English' have as much Saxon, Angle and Celtic blood as Wales or Scotland. We're all a mix of everything.
But as for a larger 'Celtic' block culturally, you need the Saxons perhaps to become more divided, arguing over conquered lands so they are less united and aren't able to push the current occupants back. I don't think you'll get much British unity because there wasn't during the Roman occupation, and with them gone you have those following Roman-British traditions, Celtic traditions, some embracing the newcomers etc. I would think you need the Saxons more divided than the British more united.
Edit:
Don't forget you have ancient rivalries between the British tribes anyway, and this will play a factor keeping the British divided. Also since the Romans wiped out the Druids there is now inter-tribe Judicial System amongst those who still consider themselves Celts so unity is near impossible. The old system is broken and the new system has collapsed in the face of the Roman Empire failing.