Well, Marc, Canada has never been part of the U.K., even before Confederation which occurred in 1867, and "Can-UK" was never a designation applied to Canadian troops in either of the World Wars. Canadian troops did use British equipment and wear British uniforms, and Canadian divisions and Corps did operate as part of higher level British formations.

I have always associated Canuck with North America, rather than Britain, so I would not have expected Brits to use the term. Perhaps some of the Brits can confirm or deny this.