I'm 5'7, a regular size 12 and I have real difficulty finding dresses, especially work dresses, that don't cling to every bump and look like a sack of potatoes on! I like Hobbs and Oliver Bonas generally as they sometimes have A-Line style / pleated style skirts and dresses that are flattering. But they are so hard to find!
I tried on a tweedy dress in Next earlier, and oh my, it looked AWFUL.