You ever noticed that their immigrants were all former British subjects from their colonies? No?
They're just going home.
That's why they have Jamaicans in Britain, and not folks from Martinique or the Dominican Republic. That's why France has immigrants from le Morocc and le Algerique but not Jordan and Turkey. That's why when people talk about Asian immigrants in Britain, they think people from the Indian subcontinent, not Chinese or Koreans. Say Asian in the USA, and what do people think of?
You sure didn't say no three centuries ago, not that you could then, nor could you do so now.
40 years from now, when Scotland and Wales and Upper Northwest North Umbirngton are no longer part of Britain, and all that's left of England is basically greater London, with the motto, "Well at least we're still bigger than the Isle of Wight" y'all will still be migrating to it, if there's anything worth migrating to. London will be down to one bank and a post office.