On closer inspection, this has to do with the way USPS labels the UK. I would expect it to be listed as the United Kingdom, but for some reason their dropdown shows "Great Britain and Northern Ireland".

Try going to Zen Cart Admin > Locations/Taxes > Countries > United Kingdom (on the 12th page), and change that to say "Great Britain". You could also try "United Kingdom (Great Britain)"

Let me know if that works. It would be a temporary fix until USPS changes their label for the UK. Another option is to modify the code so it "translates" the country label for the UK. I will consider adding this to the next release.