J. Mendel Black Shift Dress, size 12? What is this UK sizing??

J. Mendel Black Shift Dress, size 12? What is this UK sizing?? Looks like an 8 MAX. I need to measure this. Like new - but the zipper is SO stiff. I need to figure out what you can do to fix this. AND I need to take measurements. Promise not to buy this if I haven't updated this listing, ok??