Americans have always struggled with the more far-flung fields of the British Isles. With famous voices from Wales, Northern Ireland and Scotland frequently donning silly 'English' accents across the pond so they might stand a chance of being interpreted by Uncle Sam's viewers.
And some accents are harder than others. Glasgow, for instance, is widely accepted as 'on the thick side' even for the British public, so when famous Hollywood actor James McAvoy came on Saturday Night Live to show off his native Weegie pipes, American audiences all over the country were stunned at how difficult they found it to understand him. James McAvoy, known in America for roles such as the X-Men patriarch 'Professor X', who speaks with a typical English accent, and the dissociative identities of M. Night Shyamalan's Split, most of whom are American.
But McAvoy, is actually from Glasgow, and an underserved, working class area at that (bucking the trend of upper-middle class, privately educated British actors making it in Hollywood) - McAvoy's accent on SNL is entirely natural.