Hm... I think a POD in the Civil War will work. Make the UK help the CSA a little more conspicuously, but not to full war. Irish immigration is met with open arms from vengeful Americans. Immigration speeds up, as does industrialization in the North. Eventually, have US declare war on Britain in the 1890s/1900. War lasts until, say 1907 with the US seizing most of Canada (eating and then spitting out Quebec), several Caribbean islands, Ireland, and Scotland. Since ASBs are now rampaging throughout Europe, teenage freedom fighters Hitler and Stalin team up against the American imperialists. A general war begins in Europe, with no one really understanding what's going on. Amid the chaos, America convinces England that they themselves have always been an American colony, not vice versa. President Roosevelt, in a famous quote, says "Yeah, that was a typo in some school books, stranger things have happened." He was roundly agreed with by the shrugging populace. By 1975, Wales, England, Scotland and Ireland are all states (Ireland not needing 2 states because the gov't doesn't care about religion anyways.)