It's a great dress and perfect not beach or casual, just a cool stylish day dress that will look lovely in summer in London.
As for styling, flat sandals and a straw bag? Sunglasses, Trainers in a bag? Bouncy blow dry? Dewey make up?
I would wear it with my new palmiera style sandals as they are the most comfortable shoes ever.
I want to go for tea a the titz!
Congrats on the weight loss OP enjoy your fabulous occasion.