^ seems to be the only person in this thread who knows what he's talking about.
The Bretons are a fusion of real-world Celtic and French because they are Celtic, overrun by French culture, in real life - Brittany is France's Wales, to oversimplify things.
Really, the Breton accent would be best
The question is what is a "Breton accent"? There is no strong accent in Brittany, at least not where I live (Nantes), people pretty much have a standard French accent, like what you hear on national tv. Maybe in some remote parts of Brittany there is a stronger accent, though (my brother in Rennes hasn't run into strong local accents, either. Maybe in villages?).
If you mean the accent of people who speak the Breton language, then I really have no idea what the result would be. I have never met a Breton speaker who spoke French with an accent (there are probably a few in western Brittany, though), I figure most of them would speak English with a French accent. Maybe some kind of mash-up could work to make the Bretons in Skyrim stand out: a French and Breton accent with some Breton words thrown in. I'm not sure what the result would be.
It would probably be easier to use voice actors from Wales (or Ireland or any country/region where people speak English and a Celtic language), though. (edit: that is if you only want to keep a Celtic touch in the accent; you would lose the "French accent" bit we've been talking about in this thread) .