America and Britain share a common language, culture, heritage, etc.
What I think is that when a group of people perceive that there is no chance of they themselves sitting at the top of the heap, they choose to follow an alternative rather than even joining those at the top because they just see it as being second fiddle no matter what.
Especially when those people are Jewish because they are Q”God's chosen people” and the Brits have royalty and see themselves as an extension of royalty over the rest of us peasants on the planet.
Not all of them though but I do think that is the gist of it when it comes down to those types.
“Why are there Anti-American Brits?”
Same reason there are anti-American Americans. A media that tells them “America bad”.