Get as close to the mm sizes as you can, the old UK sizes aren't used much anymore, they generally go by the mms now. Not all the mm sizes have US equivalents, so get the closest you can.
But what I'm also saying is that even if you did have the correct size, you might not be able to match the pattern gauge with it, and would have to go larger or smaller. This is normal as the designer may knit or crochet looser or tighter than you. Use whatever size gives you the gauge for the pattern. Same thing with matching the yarn, find some that is usually worked at the gauge as the original on the needles or hooks.
sue- knitting heretic