Laura, KM runs small. Also - don't confuse the UK sizes with the US sizes. A UK8 is a US2.
I wear a US0 most of the time (a UK6). A US00 from places like WHBM, J.Crew, Ann Taylor, and the Loft because their fits are not narrow. But in KM and Boden - I wear a US2, which is their smallest size. And it fits like a US0.
What US size do you usually wear, and from which store?