By post WWI I mean the 1920s. The US had entered the war without a real standing army, and exited it with a large force, battle-hardened, and one of the biggest navies in the world. At that point, various US/British clashes like the war of 1812 didn't seem to be all THAT far back in the past, so Britain was a trifle worried about what the US might try.
By the time WWII was actually well underway, the US was pretty much the only thing keeping Britain in the war. There were no anti-US plans at that point, obviously, since that would basically spell the defeat of the UK.
The general attitude in the US at the time was "It's Europe's problem. They started it, AGAIN, let them sort it out."
Frankly, if Mexico and Canada had declared war on the US and invaded, I doubt that Britain would have as much as batted an eye over it.
It took Pearl Harbor to convince the US public that we weren't going to be able to sit this one out.