I expect it to be very British and French, I wouldn't expect to see knights wearing glamorous armor sets as Bretons don't get any racial attribute bonuses connected to melee weapons, armors or smithing. I'd find it hard to believe this would be a major factor to their homeland life.
Bretons are the least of the human races likely to produce good armor sets but are best with magic.
Mages towers is a must
You are away off the mark here, actually. Again, I should point you to the Pocket Guides, in particular http://www.imperial-library.info/content/pocket-guide-empire-first-edition-high-rock. The uniting idea behind High Rock is feudalism X 100. Preposterous knightly orders, endless questing and adventuring, layers upon layers of chivalric obligations and oaths. An ecstasy of Arthurian fantasy. On top of that, we have this ancient mage-class, that is highly important to Brettic society. Somewhere between the medieval artisan class and the clergy, these guys are omnipresent, collected into numerous guilds and societies - the greatest of which being the Imperial Mage's Guild. Essentially though, the Bretons are a race of proud, haughty knights in gleaming armour, puffing out their chests as they march about in horses draqed with the most gaudy heraldic designs one could imagine.
I cringe when people say "High rock is basically a mix of French and English" the way I see it, High rock is more a mix of all European cultures squeezed into small area. I think the Illiac bay is actually more mediterranean in Flora and Fauna, with wild cats and wild hogs replacing wolves, (Don't forget, a quick hop across the pond and you get deserts) The Ayleid ruins add a small sort of Greek flavour to places like Mages Towers and Temples.
The North East on the other hand is more like Skyrim, or Scotland I guess, There are Elks, not like the Skyrim deer, more like the Irish Elk of the Ice age and Large varieties of Wild Boar. The North East and the north coast is mostly where the forests are concentrated. some of the border towns show influence from other cultures who invaded, Jehenna and Farrun show a slight Nordic mix, Evermore is like a smaller, more mountainous version of Wayrest.
The wild North is very different from the cosmopolitan South. The Bretons of the North resemble the Ancient Gauls with their hill forts and their round houses, their murals, Plaid designed clothing and swirling patterns. They have a lot more respect for magic users as well, revering their Shamans the way the Ashlanders of Morrowind revere their Wild Women. The cosmopolitan Illiac bay cities more resemble medieval Italian or German cities, with vast mages guilds in each of the major ones.
The Hrothgardian mountains resemble the Eastern Steppes, with wild horses and large rodent life, (Some kind of creature resembling Marmots)
If anyone has played Rome Total War, the default Celtic/Germanic/Barbarian cities are what spring to mind when describing the northern cities, while Assassins Creed is what comes to mind when describing the Illiac bay cities.
Obviously this comes all from my own head, I just think if someone were to make a mod or a game set in High Rock, my version will offer the most variety and would be the most interesting.
I think you've got most of this right here, but are a little off in your original assessment. Frankish culture does seem to be the model for Daggerfall, Camlorn and much of the Iliac. One thing good to keep in mind, is although the climate and the peasantry is likely to vary wildly as you say it does, the nobility will generally be intermarried, and share a High Rockian culture. The King of Daggerfall and, for example, the Queen of Evermore would generally have more in common with eachother than they would with even their own middle class, or the different ethnic groups that may reside in the eastern kingdoms.
As someone who has had to struggle with the mercilessly small amount of lore on Evermore in particular, I can vouch that (depending on where the map puts it), a large amount of its populace would be Bjoulsae, and likely not even speak the same language as the city-dwelling folk. The best way to understand High Rock, I think, is as many contradictions rolled into one as possible.
One last thing, whatever you do don't confuse North East High Rock with North West
. Don't forget that the majority of The Reach is within the borders of Highrock, so that is were you would imagine most of the Druidy Gaulish stuff goes on.