Zappos.com has a shoe size conversion table available on their website. The UK sizes can be found on the third column to the right, with the corresponding US size on that same row.
The website contains tables for men, women, big kids, little kids, toddlers and infants, including conversions for US and UK, as well as European sizes. The tables are based on converting US to UK shoe sizes, but can easily be read to convert UK to US shoe sizes. The tables provide inch and centimeter measurements as well, and at the bottom, European-sized brands are listed. As a rule of thumb, US men's shoes are half a size larger than UK men's shoes while US women's shoes are two sizes larger than UK women's shoes.