So, my question is kind of complicated to put down but during the early Roman conquest of Britannia there were quite a few tribes located all over the south of the isle but then they spend about 350 years under Roman governance (along with the putting down of at least two major rebellions). These tribes were then displaced westwards by the Angles, Saxons and Jutes.
This brings me to my point: Were the "Bretons" spared the conquest (also was it genocide, assimilation or aristocratic takeover that led to the Bretons no longer existing in the greater part of southern Britian?) in Cornwall and Wales a hodge podge of all the tribes mixed together or was there a cultural continuance of Dummonoi to Breton to Cornish in Cornwall (and the same in Wales though I can't remember the tribal names in Brittannia Secunda)?
I sort-of answered your question here, although with a little more focus on the Anglo-Saxons. Current DNA studies show that approximately 30% of the British population is of Anglo-Saxon origin, with a greater density on the East Coast and in the North because of a conflation with Danelaw settlers, and a lower density in the West, especially in Cornwall. Accounting for successive waves of migration to the British Isles, this suggests that while there was a relatively large ingress of Angles, Saxons and Jutes, a majority of the population remained Brythonic, integrating with their new rulers rather than being massacred or forced into exile like Gildas or Procopius would have us believe.
Both language and material culture appear to have assimilated each other slowly over the course of approximately a century. Grave goods excavations illustrate the creeping presence of Anglo-Saxon goods and designs westward, but also the incorporation of some Brythonic design elements. Linguistically, a period of widespread bilingualism is hypothesised, largely in central areas of the country between the Germanic-speaking East Coast and the Brythonic-speaking West, slowly declining as Old English became the norm. Curiously, Old English appears to be a hybrid of Brythonic and Germanic languages. Despite an overtly Germanic lexicon, OE has a series of Brythonic pronounciations and grammatical structures which would have made it sound quite different to the Germanic spoken by the Saxons in the "Old Kingdom". A very similar thing actually happens following the Norman Conquest of 1066 with proto-Middle English developing surprisungly quickly from an amalgamation of Old English and a melting pot of French dialects.
So to answer your question more directly, the Britons in Wales and Cornwall are most probably a continuation of earlier Brythonic tribes rather than amalgamations, since the bulk of the Romano-British population seems to have stayed and assimilated with the incoming Anglo-Saxons rather than fled.