I have looked on Isabella Oliver and Seraphine and can't find anything I am 100% about. I think I'd need to order a few things to try on.
I don't want anything that looks frumpy, so think something more fitted would be better, but I am also a size 16 with very large norks, so don't want anything that will look ridiculous. I'd prefer either my arms to be covered or to be able to wear a blazer or jacket with it to cover my arms.
I bought this recently.. I also like my arms covered. It is really flattering. I think I will get a lot of use out of it. HTH .. enjoy the day.http://www.cravematernity.co.uk/collection/dresses/keyhole-back-dress.html
I'm a similar shape to you, and I have found that the IO - style fitted dresses look positively pornographic with my bosoms. I've had better luck with non-maternity dresses in suitable shapes, and I am still comfortably fitting into a couple of Jaeger tunic dresses at 28 weeks with plenty of room for bump.
I had a wedding at 8 months pg and looked everywhere for a nice dress, ordered loads and sent them back, eventually found a really nice one in www.jojomamanbebe.co.uk/ It was last season so don't know what they have in now but might be worth a try. If you live in north london they have a clearance shop on finchely road that often has nice stuff in....
I wore this one to a christening last week. It's quite low but you could put a vest or cami under it. Also size 16 with big boobs. Accessorised with printed pashmina, pink and coral jewellery, nude shoes.