I have worn silky trousers with a vest / shell top and a floaty shirt / blouse + pashmina for warmth if needed. This of course may not suit a formal wedding, but my sons were fine with it and it suited me much better than the usual 'wedding outfit'. In our village church it would be very acceptable, indeed what most of us would wear.
You may well be fine to try buying online for a 'forgiving' outfit such as this. And I would definitely look around the good charity shops, and some of the cheaper High St. stores. My trousers were one of the posher shades of beige which allowed a colourful top.
I also suggest online trying La Redoute who have a great range, not the cheapest, but not too expensive either. I don't know exactly what 'low' means! And I would look at Oxfam online as well - more difficult with sizes but I think worth a try, these kind of clothes have a lot of 'give'!
If you're not used to buying online, don't be tempted by some of the lovely looking clothes from the Far East - a few people have good stories to tell, but mostly not!
And get opinions from a trusted friend.
I just decided to have a look at Oxfam online for myself, and saw this:
www.oxfam.org.uk/shop/womens-clothing/dresses/decisions-size-14-multi-coloured-full-length-2-piece-dress-hd_101179082
Cost £21.99, and would look lovely with a pashmina (cheaper and more flexible than a jacket)