The British Empire was the United States' first enemy and many in America's founding generation were severe Anglophobiacs. This animus continues for some time in much milder form until the early 20th Century, I remember reading that FDR once told Churchill that Americans had a natural distrust of the British, seeing them as "Imperialists". However by the middle of the century until today, the US and the UK were locked in a "special relationship", but what POD's (probably from the early to mid-19th Century) could perpetuate the "Anglo-American Animus" well into the 20th Century?
Maybe one where the Oregon territorial dispute remains unresolved which leads to President Polk running for a second term in which he goes to war with the British over the territory. The most common one however seems to be British support for the Confederacy during the ACW, but what other scenarios are possible?