A UK size small 8/10 would approximate to a US size 4/6, that much I know.
A UK medium (12-14), is like a medium here, an 8/10/12.
Problem is, it may not be so much bust size as back width. Example, I take a US size 6 on top (UK size 10), but would go for a medium for lower half, size 8 here (UK 12).
Does this help? Does the 5th element site have a sizes chart anywhere?
Clare