Sort of the same kind of thing you'd wear to a wedding I think, but maybe not a hat. Smart, formal but feminine (I hate suits on women at weddings).
I have no idea what is in the shops as have been in maternity jeans for months, but am similarish build to you, and usually find something in Monsoon, Hobbs, Jigsaw or Phase 8 (when I am feeling flush). But Primark and Peacocks can have some gems from time to time if you go to a big branch. Also, you never know what you'll find in TKMaxx (Apart from tiny jeans in dodgy finishes).
I agree about the Coast dress. It's my favourite too.
I know it's quite a bit more expensive than the others but when you consider how much wear you'd get out of it then it's a good investment. Navy works in both cold and warm months. It would be a great dress for an informal Christmas/New Year party but would also work for a wedding.
Wrap dresses are so versatile if you choose the right colour.
Had a quick look on Ebay at Coast wrap dresses...
This is gorgeous but a 12. Could you wear a 12 as it's stretchy fabric? You could put a thin vest top underneath if it gapes.