Ulla Popken is a great store for finding clothing in larger sizes. They are also quite good in stocking smart/formal wear. These 'Frock coats' are long-length blazers which would be perfect with a skirt (for taller women) or wide leg trousers if you are more petite. Prices start at £31, click on each to buy:
They also have a selection of longer length blouses. I love this one in pink, and it's on sale now for £17. I've paired it with a black skirt here, but it would look great with grey, navy or brown too:
