True, Britons were successfull on certain occastions, but got their bums spanked by the Saxons time and time again, mostly due to tactics and equipment. There's a reason why Briton Gaelic is only spoken in Wales and to a degree in Kent.

(And yes the saxons in turn nearly got spanked by the vikings, and later GOT spanked by normans.)
And of course the Lannisters have their share of panicking aswell. As I stated in my previous post, most battles in Westeros is determined by aforementioned reasons, meaning that any Westerosi army properly led and positioned can have victory. They were desperetly trying to fend off Gregor's surprise assualt, and proving unsuccessfully doing so (which leads to panic/routing), they were destroyed. Had they known/been better prepared (as when Edmure Tully defended the Stone Mill crossings) and been on the other side of the river, I'm sure the outcome would have been diffrent.
And yes I have, but then in Paolini's series, there's dragons, mages, magicial species and medieval equivilents of terminators. I'd hardly call that warfare medieval
