I looked at the USPS documentation again, and I noticed that the country is given as the actual name, not the 2 letter code. So UK is listed as United Kingdom in the uc_countries table. The USPS docs provide a list of all the valid countries that they will deliver to: http://pe.usps.com/text/Imm/immctry.htm. "United Kingdom" is listed there as a subcategory of Great Britain. Since they're both listed, I don't know if anything needs to be changed or not.