I'm a UK 8 and a Euro 42 as well, Caro. In American sizes, I find it really depends on the style of the shoe and the brand. If the shoe is closed and has a narrow toe box, I often find the 11 to be a better fit. If the shoe has an open back, is a sandal, or has a wide toe box, then I'll buy a 10. I think the actual corresponding US size would be a 10.5 which is not available in most brands. To complicate the issue even further, I find US sizing can vary quite a bit from brand to brand--in some brands, I can hardly squeeze my foot into a size 10, while, in other brands, the size 11 is enormous.