When I was looking for bridemaids dresses earlier this year, we searched everywhere and House of Fraser ended up having the best selection. They have a collection called JS Collections which is really nice quality for around the price you're looking at. Some are a bit prommy/OTT, and others don't look like much on the website, but the ones that are heavy jersey are beautiful, worth a look.
Not an immediately obvious option perhaps, but I got a stunning dress from an ebay store called Dress it Plus Size
The name is a touch misleading, as they cater for dress sizes UK 6-20 and they provide a sizing chart on each dress for you to check your measurements against, which for me matched my usual UK dress size.
Although they are based in China, I got my dress in under 10 days and was so pleased I took a punt on it, as the dress is beautiful, great quality, good value and I've had so many compliments on it.