Ahh, so you want to lock down the border between the Republic of Ireland and the United Kingdom, because if you drop 1 million citizens of Ireland into London they also don't simply become British but are just Irish living in London.

The Welsh might have a comment about how much the British government tried to erase the native culture and people who lived on those isles for thousands of years before various foreigners came in on little boats. How are the native Cornish speakers doing these days?

The American Empire grew in no little part by specifically importing people from Africa to work as slaves.