Hey There! I just learned about this new brand called Universal Standard, seems like a beautiful collection of higher quality products. The coat above and its little red detail tell it all (although the campaign is a little too subtle and dark, I cannot really take a good look at the overall craftsmanship of the collection) , but you can be the judge and below is the rest of the collection.
Talking about work clothes for plus size women, just like any other plus size fashion or clothing, there aren't too many choices out there, but of course you still have some options and it's not like you won't find clothes to wear ;)
The best established brand goes to Carmakoma from Denmark for it's elegant design that fits properly on plus size women.
The spanish brand was one of the first to use a plus size model in their campaign back in 2007 ( I still have the hard copy booklet!), but not until last year, they launch the plus size line Violeta. Just like Mango, it covers all segment for plus size women - casual wear, party wear and also included workwear.
There are also other brands which are not as minimalist as could be, but good options for the formal dressing needs.