The answer is that it was. The Brythonic kingdom of Dumnonia thrived on the south west peninsula of Britain from the 4th to late 8th centuries, in the area of Devon and Cornwall. This “West Wales” represented a maritime trading giant during this period, thriving in the post-Roman vacuum and establishing formidable resistance to the expansion of the kingdom of Wessex. Eventually, the Anglo-Saxons were able to defeat the Devon part of Dumnonia, but Cornwall retained much of its independence – using the Tamar River as a natural defensive border – for many years afterwards.
While the Brythonic language faded in Devon, it continued to exist for centuries in Cornwall. Despite numerous efforts to declare the language dead, there is evidence that it persisted, and it is now part of a revival movement.
Cornwall had a Stannary Parliament with authority to govern its own matters. The deliberative body was not addressed in the 1707 Acts of Union, causing many Cornish today to claim that Cornwall continues as an independent duchy not unlike the Isle of Man and the Channel Islands – not under the rule of the London, but rather a crown dominion. The argument certainly fails any reasonable legal or political test – so far – but the on-going effort of Cornwall to maintain its independence persists. Cornwall is, simply, a long way from London, and with a river cutting it off from most land-based access, it is nearly an island in itself.
To follow up on the response given by u/itsallfolklore, I covered a little of what can be said about the independent rulers of Cornwall – Dumnonia – during the Saxon period in an earlier post, below. Our biggest issue from this perspective is that no writing from inside the kingdom survives (other than inscriptions on a few monuments). We know about these people solely from the archaeology, and from the passing references made to them by their enemies, the Saxons.
What do we know about the political situation or Rulers in Cornwall in the 700s-800s AD?