For folks in the USA, sending cards, letters, packages, etc. to United Kingdom:

For YEARS, I have use the abbreviation: U.K.

Was at the post office this morning, had one package to ship to U.K. The Postal Clerk took a marker, blanked that out, and wrote out "United Kingdom." I said, "Don't like my abbreviation?" Reply:

It's the computer. "UK" means Ukraine, send something there and you'll never get it back.

So be sure to write it out completely!

I checked the card exchange, didn't see any issues on that list.