Well bangles and helmets and their ornaments aside, the Bretonnians wear pretty standard transitional plate armor. A mix of splint, a breastplate under their tabards, and chainmail. Perfectly serviceable, if a bit old, but not like Beastmen and Orcs can make much better. The fact that TWW added barbutes and other such helmets for Knights Errant suggest they may wear them under their greathelms, which I think was a historical thing.
The big reason they probably don't use Empire-style designs is because they don't have access to them. The Empire has benefited from a long relationship with the Dwarves. The Bretonnians only made some friendship with them after the Crusades, and even then it was definitely not as close as the one Sigmar and Kurgan forged. The fact that they've gotten to the level that they're at, essentially on their own, is a bit admirable, considering it looks like back in Gilles day everyone was wearing chainmail hauberks alone. But Brets being Brets, they probably consider it a point of cultural pride to be so equipped and to not have to rely on armor as much as skill at arms and the Lady's blessing.
Oh, and the helmet ornaments are probably heraldic, and there's at least an option for a knight to take the Antlers of the Great Hunt as a magic item, so maybe they're also a badge of honor and maybe a religious item too.