I agree with jahenders -

We don't know how the Romano-British elites of the time viewed themselves - as Romans who lived in Britain, as (Romanised) Britons or as Romanised Trinovantes, Iceni etc. Once the 'Roman yoke' was lifted (or at least its tax burden and bureaucracy) the basic unit of government may have gone back to the
civitate or tribal capital plus surrounds. I think some 'Dark Age' historians have commented that many of the early Anglo-Saxon kingdoms can be seen as successor states to the pre-Roman conquest tribal kingdoms or the civitates. (Essex=Trinovantian lands, Kent= Cantii etc.) This suggests they may have fought amongst each, hiring mercenaries that later took over the kingdoms for themselves or married into the British aristocracy.
Gildas does see "Britons" as a race separate from the Saxons but the struggles in the 5th century may not simply have been British v invaders.
Worlds of Arthur by Guy Halsall is an interesting look at the period. There are of course lots of others, both popular or academic.
http://www.goodreads.com/book/show/16132408-worlds-of-arthur