Suppose it would depend on the type of site you were doing it on, if it was something you were marketing on and offline you certainly wouldnt want joe blogs to confuse my shoes.co/uk with shoes.co.uk who potentially would be a competitor.
Or if it was a site which targeted multiple countries from the same domain each targeting its own sub domain or folder then it would be justifiable.
If its for the sole purpose of adding a UK parameter into the URL and you were relying on PPC or natural search so people didnt really pay attention to the URL then..........it seems a bit pointless as there are ways to gio target with out the usage of an extra couple of characters on the end of the domain imo.