lucy24 - 10:57 pm on Feb 22, 2013 (gmt 0)
If I see a UK flag I know that the page will be translated into British English, which will contain different spellings and usages than if the link had been a US flag.
And if a unilingual user doesn't recognize the Union Jack, what are they doing on a non-English page? (If the page is already in English, they will not need the link.)
If your site is based in Spain, use UK English. If it is based in Mexico, use US English. Google translate may not know the difference, but humans do.