I made 2-3 orders from Boohoo. Depending on the clothing item, I usually wear small or sizes 4-6 US (UK 8-10). However, the dresses I got from Boohoo, I would personally go for size US 6 (UK 10).
Shipping was good. I'm from Montreal, Canada and my packages normally arrived within a week and if you order over $50 then you don't pay duties or shipping, which is pretty good!