Yeah, Bede isn't a great source for early medieval period warfare in Britain. The most descriptive sources are Celtic. St. Gildas' "De Excidio et Conquestu Britanniae" was quite good, and expounded greatly, if obscurely, on the why of things during the period covered.

The Historia Brittonum is less useful, more clogged with mythological bullshit but is still work a browse or two.

also, here's a link to the Book of Aneirin in English, the poem "Y Gododdin" will be of most use to you, it is mainly tributes to Brythonic warriors who died in battle against the Saxons in what is now Lothian.
http://www.maryjones.us/ctexts/aindex.html