I love this dress so much. I'm a UK 18-20 and ordered the 18. It fits so well. The back is a little fiddly to get right when first putting it on as the middle wants to be pulled together permanently, but after some adjusting, it looks so nice. The shape of the dress is so flattering especially because the bungy pulls it in at the sides, and the material is almost like shapewear in the way it feels without being clingy.
I adore this dress 💞